Ok some quick corrections before I get started. The main Russia tank was not the T-26, or the T-34, which is the tank I think you actually mean (or not?), neither was the Panzer II Germany's main tank.

In 1941 at the beginning of Barbarossa, Russia's main tank was the BT-7 which was a Cavalry Tank similar in role to tanks like the American M1 Combat Car, in that they were intended to exploit holes in enemy defenses created by other combat arms. The BT-7 chassis also formed the basis for a number of over vehicles in the Soviet inventory. The T-26 was the most numerous tank in the Soviet inventory, but that does not make it the main tank.

The Soviets on the other hand had only about 992 T-34s on hand on June 22. It was intended to replace tanks like the BT-7 in Soviet inventories, but they simply didn't have time before the German attack came.

As for the Germans, the Panzer II (they had 1074) wasn't even the most numerous in their inventory and hadn't been since the invasion of France, though they were a close second, and further, the Panzer II by this time had been mostly relegated to scouting roles because while its weapons were effective against soft targets like infantry and vehicles, against the most modern Russian tanks, it was useless. Though because so many Russian tanks were the outdated T-26 and BT-7, it could still be useful.

The most numerous tank in the German inventory was the Panzer III armed with the 50mm Kw.K L/39 which was a capable weapon against older Russian tanks, but couldn't penetrate the glacis of the T-34 even at close ranges. The turret front on the Mod. 1940 and 1941 T-34 was still vulnerable to the Panzer III at medium ranges however.

The Panzer III had been intended as Germany's main tank, with the Panzer IV originally intended as an infantry support tank. The realities of the war in Russia however forced a reversal in that arrangment. The Panzer III did not have a turret ring large enough to be able to mount a larger gun than the 5cm Kw.K, though they eventually jammed a 7.5cm Kw.K L/24 in its turret. The Panzer IV on the other hand did, and so it became Germany's main tank after about mid 1942, while the Panzer III was relegated to the infantry support role, as well as a supporting role in Germany's heavy tank battalions for a time.

As for why Germany had such great initial success in Russia... well that is much more complex, and certainly more than simply a function of the thickness of the armor on their tanks, or the size of their guns. The invasion of France is a perfect example of that because on paper the latest French tanks were superior to their German adversaries, but a combination of limited availability of these new tanks, strategic surprise, and excellent German organization and tactics meant that the on paper superiority of French tanks meant very little.

So lets break it the campaign in Russia down a bit so we can understand some of the reasons why it was so successful. I will be painting the campaign in rather broad strokes but I will try to touch on all the most important reasons why Germany did so well.

First lets look at the organization of the German and Soviet armies, because this was one very important factor in why the Russians struggled early on.

In 1941 the Red Army was undergoing a massive reorganization directed in part by the competent, though often unlucky General Timoshenko^1 who advocated for the mechanization of the Red Army. In 1940 the Russians had begun creating large Mechanized Corps, which were supposed to have about 1031 tanks and 36,080^2 men. But in 1941, only a few were at nearly full strength. On top of that these massive formations were rather unwieldy and one dimensional, and with the chaotic state of the Russia command system at the beginning of the war, these formations we rendered largely ineffective and were quite often destroyed to a greater or lesser extent. The Corps consisted of two tank and one mechanized division, but even that was a tank heavy division. Further the whole formation simply was given enough dedicated logistical to make it function. And to make matters worse the various pieces of the Corps were scattered along the entire border between Germany and Russia, each piece often being around 60 miles apart^2, which dispersed their strength and made them difficult to manage.

To give some context, a Panzer Division at full strength would have between 150 and 300 tanks, had about five attached infantry battalions, and would usually be attached to an Army Corps made up of other combat arms.

The other formation which was found to be rather unsuccessful was the Rifle Corps, which like the Mechanized Corps was made up of one type of division, in this case Rifle Divisions. Though while on paper these divisions would have numbered around 14,485^2, these divisions rarely had more than 10,000 men and quite often much less. To compound the manpower issues the Rifle Corps were given little dedicated logistical support which meant that maintenance support had to come from maintenance battalions attached to Army commands.

The Soviets attempted to solve to manpower issue by calling up 800,000 reservists in the spring of 1941, but they were just beginning to reach their assigned units when the attack came.

Lets also not forget the purges conducted by Stalin in the 30's which gutted the Red Army's officer corps.

Surprise and concentration of forces was also a key factor in Germany's initial victories.

At the beginning of the war, despite Russia having a larger army than Germany, its forces were dispersed all over Russia, much of it in the east protecting against Japan. That meant that while the Red Army outnumbered the Wehrmacht in total strength, the Wehrmacht was able to place more men on Russia's western boarder than they were^3. On top of that the forces on the boarder were still in the process of fortifying the new boarder with Germany. After the Russians had moved into Poland, they had mostly abandoned and dismantled the formidable fortifications that had run along its shared boarder with Poland, and the new defenses weren't anywhere near ready.

The VVS had also been left is disarray by the purges, and relied on mostly obsolete aircraft and bad doctrine.

To make all this much worse, the Germans achieved almost total military and political surprise with their attack. The signs were there that it was coming, and indeed Stalin had been specifically warned of the coming attack, but he chose to ignore the signs or disregarded them as tricks^3. And because he was so desperate to delay a conflict, he tended to err to far on the side of caution when it came to dealing with the Germans^2. As a result the Red Army simply wasn't prepared for the onslaught and suffered accordingly.

Lastly I want to briefly cover the Germans.

In 1941 the Wehrmacht was at the height of its power, it had defeat Poland, the French, kicked the British out of Europe. It had successfully subjugated much of Europe, and it was riding high, and seemed to many to be invincible. They were excellent at concentrating their forces and creating breakthroughs in enemy defenses which allowed them to outflank and encircle opposing armies. Their officer corps was experienced and motivated, and so were their men, and this meant that at least early on the Germans had a vast advantage in command and control, which allowed their divisions to run circles around Russian forces in the summer. At that point the Germans simply had a well organized, well motivated, and mostly well lead military. And it allowed them to do incredible things in the coming campaign.

The combination of surprise, an officer corps still trying to find its way after the purges, a massive reorganization, unwieldy and widely dispersed formations, poor logistical organization, ill prepared border defenses, mostly outdated tanks, and the German army at the top of its game all combined to ensure that the initial phases of the war was nothing more than a disaster for the Russians.

Its important to keep in mind however that Barbarossa was still a failure for the Germans. The Germans failed to accomplish the three things they had set out to do, that is take Moscow, take Leningrad, and destroy the Russian army. Once that initial attack failed, while the ultimate defeat of Germany was not guaranteed, the ultimate defeat of Russia was really no longer in the cards. The Red Army, despite all its faults, had weathered the storm and would continue to grow stronger and stronger each year.

1. David Glantz, Kharkov 1942

2. David Glantz, Before Stalingrad

3. Alan Clark, Barbarossa: The Russian-German Conflict 1941-45