This is a question that gets debated ad nauseum, and you will hear countless opinions in regards to, but there is no hard and fast answer. What I will say is that, the defeat of Nazi Germany was very much a joint effort, and anyone any claim that attempts to evaluate the potential of any one power fighting against the other without considering the effects of the other powers, should be treated with suspicion. Perhaps the USSR didn't need the US... or maybe Lend-Lease was necessary for victory. Or the air campaign by the Western Allies which contributed to Soviet air superiority. Or the blockades. Or the dozens of divisions that had to be kept away from the East even before the Allies landed in Europe. Or could the Western Allies have landed in Normandy and fought through France had the USSR not been holding down the majority of German divisions? You get the point... There are a ton of factors in play, and concentrating on just one (such as the disproportionate Soviet casualties) doesn't allow us to see the whole picture very well.

So what I am going to say is that a good place to start is in evaluating the economic potential of the two powers, and the impact that one had on the other. To that effect, I would recommend you check out /u/iHistorian's answer here, his and /u/Elm11's responses here, or my answer here

European front* gotta put that in there.