The main reason behind casualty disparity between the Red Army and Wermacht throughout most of the war can be mostly explained by German firepower advantage over Soviet forces both in terms of sheer tonnage of ammunition used and in terms of tactical mobility and flexibility of artillery.

For example Red Army expended ~446 thousand tons of artillery shells in 1942 or about ~1222 tons per day total or ~3 tons a day per standard division.

Let’s compare it to German figures: ~710 thousand tons of artillery shells were expended by Wermacht in 1942 or about ~1900 tons per day total or 10,5 tons a day per standard division.

So Germans had almost double advantage in sheer ammunition expenditure and used three times as much ammo per division in comparison to Soviets.

This disparity is what skewed the casualty ratios the most. Soviets had to find ways to deal with superior German firepower. And the primary method to do was to perform multiple simultaneous attacks tactically and offensives operationally to prevent Germans from concentrating their artillery to defeat Soviets attacks in detail. But consequence of such method was that at least some Soviet units always suffered lopsided casualties.

Sources:

• Waffen und Geheimwaffen des deutschen Heeres 1933-1945 by Fritz Hahn.

• Main Artillery Directorate (GAU) reports via Alexey Isaev.

P.S. I must note that Germans suffered more casualties than the Soviets in Seelow heights. Soviets lost about 5-6 thousand KIA in fighting at Seelow Heights versus German irrecoverable losses of ~12 thousands.

P.P.S. If somebody needs more raw data for their own conclusion: there is a compiled table made by Igor Kurtukov and translated to English by me. https://docs.google.com/spreadsheets/d/1-73qbzhLv1UT3LeQNGfUfuR3YFGEW7UtRZPkdlBTAmw/edit#gid=0

This is a very complicated and somewhat controversial topic. Shortly after WW II the reasons for the Soviet victory over Nazi Germany from a Western perspective were heavily influenced by English and German writers and biographers who tended to overrate German qualitative performance and underrate Soviet performance. In the present day I would argue that this way of thinking has been prone to overcorrection, particularly by amateur internet historians who unfairly discount real German advantages that they held in doctrine, tactics, and overall combat efficiency over the Soviet armies.

All that being said, I would argue that the USSR's advantages in men and material (which included hefty injections of material from allies) were the decisive element in their victory. While Soviet Deep Battle doctrine was largely sound, forward thinking, and effective, I cannot think of a source that attributes this particular doctrine as a primary reason for Soviet victory.

Looking at the war on the Eastern Front and the resources available to each side, I don't think the outcome would have been any different had the Germans adopted a deep battle concept and the Russians a more flexible blitzkrieg like concept. Both had much in common in terms of an emphasis on combined arms, breaking through the enemy lines, and thrusting deep into the rear with mobile armored formations. Deep battle arguably emphasized a more centralized control over operations while German doctrine of course emphasized the officer on the ground using his initiative and at times deviating from the plan, but otherwise the doctrines seem extremely similar from my reading.

I think it's fair to say that regardless of doctrine, once the Soviet sustained the German's attempted knockout blow with Barbarossa and pushed the Germans back from the outskirts of Moscow, the chances of German victory over the USSR were extremely slim due to Soviet advantages in manpower, material, and geography. By the latter half of the war the Soviets were very good, and used deep battle to mount some very impressive operations against the Germans, Operation Uranus and Bagration come to mind.

But I think it's also fair to say that overall, the Germans displayed higher combat efficiency and professionalism than the Soviets or the Western Allies on the ground during WW II. In general for most of the war, up until perhaps late 1944/45, German soldiers on a 1:1 basis was better on average than the soldiers they were fighting. In particular better trained and better led. The primary problem for the Germans was not qualitative, or that they were confounded by an Allied army doctrine, but that they were outnumbered upwards of 10:1 after Pearl Harbor.

Now some might say, with some justification, that the Germans for the second half of the war were fighting largely on the defensive, and that you would expect the attackers to suffer worse casualties than the defenders. That is true to an extent, but it's also true that the German army remarkably retooled itself in the middle of a world war from being primarily an offensive force to a defensive one, and they were damn good at it. All while obeying the largely misguided orders of Adolph Hitler. It's also true that large offensive operations against overmatched foes can net huge numbers of prisoners (Case Yellow, Barbarossa, Operation Uranus, the battle for Singapore, etc...) if the enemy is not very good at what they do.

There are so many books about the Eastern Front it's hard to choose even a handful. Forczyk's series, Tank Warfare on the Eastern Front is a favorite of mine. Stahel also has some good deep dives on particular campaigns on the Eastern Front.

Western historians of the Eastern Front, such as John Erickson and David Glantz, have pointed to significant advantages the Wehrmacht maintained until late in the war in small-unit leadership, organization, battle tactics, weapons, and training. Max Hastings, in his work on Normandy, maintained that the Wehrmacht outperformed all of the other Allied units they faced--the U.S., the British, the Canadians, the Poles, and the French.

In leadership, the Germans emphasized quality over quantity in their officers and non-coms, and were pretty ruthless in weeding out low performers. German officers led from the front (as shown by high battle casualties even among regimental and division commanders) and unit cohesion was generally quite high. German training standards were high and their battle training was probably better than that of any allied army. The Germans also grasped and implemented the idea of combined arms (tanks, infantry, artillery, and air all working together in a coordinated fashion) earlier and more effectively. The Russians did not begin to effectively integrate tanks, artillery, infantry and airpower until mid-to-late 1943. The Russians spent the first two years of the war climbing a very steep (and bloody) learning curve. They suffered considerable deficiencies in leadership, tactics, and training, to the extent that even when they had significant numerical advantages they were often unable to exploit them effectively.

A small but telling anecdote: Robert Forcyzk, in his work on Eastern Front armor, found that before 1944 Russian tank crews were not trained to routinely zero and re-zero their guns. "Zeroing the gun" is what you do to ensure the cannon and the sights are aligned, so you can hit what you aim at. German crews were trained to do this routinely, including after every fight, after maintenance, after a road trip, etc., to ensure their guns were always zeroed. So in battle, German crews would routinely score hits with their first or second shots while Russians would be lucky to hit anything at all. Training matters.

So, the question, would the USSR have lost due to sheer attrition had they not had superior numbers, we can say, perhaps, all other things being equal. It helps to add a little context. The German General Staff and Hitler did not plan for a war of attrition--their intent in Operation Barbarossa was to fight several encirclement battles that would annihilate the bulk of the Red Army in western Russia in the first few weeks. As David Stahel et al have pointed out, this plan was fatally flawed from the beginning because the Germans drastically underestimated the size of the forces the Russians could mobilize and equip and overestimated their ability to sustain such a large-scale campaign. From the opening of the war, the Soviets were able to inflict substantial losses on the Germans, and even before the disastrous winter in front of Moscow the Germans were already facing the harsh reality that their initial campaign plan had failed, they had no plan B, and they could not replace their losses. In short, a war of attrition worked more against the Germans, despite their other advantages, than against the Russians. The Germans knew they could not afford a war of attrition against the Soviet Union but fell into one anyway--a massive, and fatal, miscalculation.

Further, unlike the Germans, the Russians did plan for a war of attrition. The Communists had a massively deep system for mobilizing manpower reserves and industrial capacity. They mobilized virtually the entire population into their war effort--something the Germans never managed to do. They also directed almost all of their industrial and resource production into their war effort--again, something the Germans never quite managed to do. The Russians from Stalin on down had their share of delusions and miscalculations, but they understood from the beginning they faced a long and bloody struggle and it would take every ounce of their collective effort to defeat the Nazis. They were under no illusions about a short war or a quick victory. Their industrial output was nothing short of astonishing--the Soviets produced more tanks, and more aircraft, than the Germans did in every year of the war--including 1941, when they had to evacuate and relocate 2/3s of their industrial plant ahead of the invading Germans!

The German campaign plan only worked as well as it did up until late 1941 because Stalin's purges, large-scale reorganization, and repositioning forces after the Polish campaign had left the Red Army in probably the worst possible condition to meet the German attack. Authors like David Stahel and Robert Citino have pointed to substantial deficiencies of the Wehrmacht's pre-war planning and grasp of strategy. Stahel argues that Barbarossa, the invasion of Russia, was lost before it began because the German General Staff really never worked out the logistic and manpower issues implied in their war aims. Had they faced the problems of time, distance, and the disparities between German and Soviet manpower, resources, and industrial capacity, they might have recognized that they were embarking on a campaign that was simply beyond their capacities, especially given that they still faced Britain in the West and were trying to hold down all of western Europe. Only the scale of the Soviet military's post-purge disabilities allowed the Wehrmacht to get as far as it did--and the fact that it got so far into Russia only ensured it would be totally destroyed in the end.

The Soviets spent 1942-43 recovering, mobilizing, and learning how to use their resources to beat the Germans. By 1944, the Red Army had achieved a level of effectiveness that made it a match for the Wehrmacht. The clearest evidence for this is in Operation Bagration, in which the Red Army faked out the German General Staff, broke German defenses with massive air, artillery, tank and infantry attacks, and effectively destroyed Army Group Center, rolling all the way from Minsk to the outskirts Warsaw in six weeks. The Soviet quantitative advantages were important, especially in air power, artillery, tanks, and trucks. But the overall manpower advantage was only about 2-1. Just as important were the strategic deceptions that led the Germans to deploy critical armored reserves and air units to other fronts, and Red Army leaders' effective use of their resources.

Commanders like Zhukov and Rokossovsky had, by 1944, figured out how to use their artillery, engineers, and infantry to open holes in German defenses, use tanks and motorized infantry for deep exploitation, and airpower to interdict the battlefield, to serve as "flying artillery," and guard the flanks of rapidly advancing armored formations. The Russians also outperformed the Germans logistically. They had learned how to use the fleets of trucks provided by Lend Lease to move troops and supplies, and to support their armored spearheads and keep them moving. As Stalin reputedly said, quantity has a quality all its own, but it's not enough just to have superior numbers; you have to know how to use them effectively. Through 1944, the Red Army hammered the Germans, broke their lines, enveloped and annihilated entire German armies, and drove deep from central and southern Russia into Poland and the Balkans, and cut off the northern German armies. That wasn't just because they had more mean and more tanks--they also had better doctrine, better tactics, better logistics, better training, better leadership.

For more on this subject, there's a substantial literature now based on research in the Soviet as well as German military archives. I recommend anything by David Glantz, for example, Glantz and Jonathan House, When Titans Clashed and Glantz, Operation Barbarossa. Also David Stahel, Operation Barbarossa and Germany's Defeat in the East, is revealing and insightful, especially for its penetrating discussion of the deficiencies of German war planning and the relative weaknesses of the Red Army (Glantz, Stumbling Colossus, is good on the latter subject as well). Also, Pritt Buttar, The Reckoning: The Defeat of Army Group South, is good on addressing the myth that Soviet victories were just a product of numerical superiority. Bagration, probably the worst defeat Germany suffered during the war, has received surprisingly little attention from Western historians. Stalin's Revenge: Operation Bagration and the Annihilation of Army Group Centre, by Anthony Tucker-Jones, is one of the relatively few works in English that covers Bagration from the Soviet viewpoint. Robert Forczyk's two volume Tank Warfare on the Eastern Front is very helpful in understanding the differences in leadership, tactics, training, and equipment between the Germans and the Soviets, and how the Soviets learned to beat the Wehrmacht.