A thing to keep in mind is that the USA never really mobilized its capacity to the standards of other nations. When you look at how many troops were engaged by the USA versus overall population, it isn't as much as you would think...we (the USA) had a huge support system that I believe people have said (correctly?) that for every front line soldier there were 10 support troops. Other nations had nowhere near such a ratio.
Russia was a powerhouse though, it was they who really won the war in Europe. Could the USA and Britain have done it alone together? Most likely, but the casualties would have been far more appalling. According to wikipedia (I know, I know, but lets just roll with it), the USA had a population of around 131 million in 1939 and during the war had a death rate of about .32% from the war. The USSR had a population of 168.5 million in 1939 and suffered just shy of 14% deaths due to the war. As you can see, the United States came out of the war far less hurt and could have absorbed many more casualties. I think the post-war estimation of Russia's capability might be too generous in their favor, but they still were a powerhouse....don't underestimate what the USA could have put into play had they fully mobilized in the same way as other nations.