The Soviets were scraping the bottle of the manpower barrel towards the end of the war, and many of their infantry formations were seriously understrength (the Germans were even worse off, though).
The reason the Soviets still could put such a large force in the field despite all the losses are partially because they did conscript people from re-conquered (and sometimes conquered, such as the Polish 1. Army) and partially because they did not have the same need for another front, extensive cross-ocean logistics nor a large Empire to police.
The British were holding down all of Africa, the Mid-East and India, having a large land front against the Japanese in Burma (in March 1944 this included 11 divisions and 3 brigades). The British also had a large navy, a huge merchant marine (both necessary to protect long overseas supply and trade routes) and a huge airforce, including a resource- logistics- and manpower-demanding strategic bomber force.
Likewise, the Americans were fighting the Pacific War, maintaining an enormous navy and the logistics to maintain all their forces (and the French forces, which used US equipment) in Europe. A huge airforce and a strategic bomber force as well as the industrial might at home to be both the arsenal and the breadbasket of democracy.
As a comparison, the Soviet Navy was mostly dormant, except for the submarine force, and many of the sailors fought on land in the famous Naval Brigades. The Soviet air force was large, but lacked a large strategic arm. Its logistics were simpler and as a whole it was less manpower dependent. The Soviets also did not need a large merchant marine nor the logistical apparatus of maintaining a globe-spanning trade and supply network.
The Soviets also only needed to fight on one front and could concentrade all their forces there.
The Soviets could produce most of what they needed, but did not need to. The US and Britain (mostly the US) supplied all the boots, most of the canned food for front rations, most of the trucks (allowing the Soviets to focus on producing tanks) and most of the locomotives and rolling stock and much of the nitrates (for ammunition production) for the Soviets, allowing the Soviets to conscript farmers and industrial workers that would otherwise be required in the field or the factory.
World war 2 was won by British stubbornness, American production and Soviet blood.
The (British) War Cabinet listed out the total allied strength in Europe on July 1, 1945: 64 American divisions, 35 British and Dominion divisions, 4 Polish divisions, and 10 German divisions. The German divisions were purely imaginary because after the mauling they received from the Russians, the surviving soldiers were in no hurry to fight. At most, the allies would have mustered 103 divisions, including 23 armoured ones. Against this force were arrayed 264 Soviet divisions, including 36 armoured. Moscow commanded 6.5 million troops – a 2:1 advantage – on the German border alone. Overall, it had 11 million men and women in uniform.
Well, pretty much the main reason is that the USA and the UK hardly went into World War II with a standing army. Even by the end they still were only able to train 93~ divisions combined in Europe by the end of the war. To compare, the USSR had well over a hundred divisions participating in just the Battle of Berlin alone.
Not to mention it went into the war with Germany with 172 divisions (though I have heard claims that they had 190 and even higher numbers).
I also think it should be mentioned that during this time period the Soviet Union had some 162-170 million people, in comparison the US had 110 million and the UK had 46 million people (Italy and France also had a similar amount). Germany had around 80 million and Japan was somewhere around 72 million.
Given the numerical advantage he had at the time, why didn't Stalin march all the way to the Atlantic?