I agree the siege of Stalingrad was around 10x the size in terms of troop numbers but look at the overall engagement of the allies on the western front following the d-day landings. Some 4.5 million troops to push against the Germans while the Russians had something in the 2 million troop range.
Moreover- look at the armaments; the Russians and the Germans were proud to measure their airforce in the thousands to as much as 20-30,000 at their peak. The US measured their airforce in the tens of thousands (and towards the very end over 100,000- having produced some 300,000 air craft through the course of the war, equal to roughly 50% of all aircraft production in the course of the war). Look at the destruction of the Ruhr valley (for example).
The Russians sustained far and away the heaviest losses, no doubt- some 11-12 million soldiers dying. But dying doesn't constitute being the greatest threat. I remember a famous quote about the purges: "Stalin didn't just take the heart out of his army, he gave it brain damage."
The Russian war machine was not as effective. It played a (fairly important) role, but the primary punishment of the nazi war machine came from the west, and was supported by the overwhelming power in both numbers and technological prowess from the US.
I say this not as some kind of US fan-boy, but rather when I was studying this at university I was struck by the efficiency and the raw manufacturing power of the US that allowed it to switch from a 'neutral' player to 'the sleeping giant'. It was the beginning of the shift from super power, to great power, to hyper power, although I think it's hard to argue with the fact that the US is now in decline.