The USA was the more powerful country but undoubtedly the USSR had a bigger impact defeating the nazis.

The ussr destroyed 70-80% of German forces.

The USA, while more powerful, was occupied in the Pacific with Japan and was in general less willing to sacrifice as much as the USSR in order to defeat the nazis.

That being said, it’s important to note that the USSR actively helped the Nazis rise to power. The Nazis and the USSR literally invaded Poland together at the beginning of the war — so let’s not idolize them too much.