> only 10%

That does not mean anything. A car's engine contains only 10% of the parts in that car.

The Americans had some resources the soviets were in short supply of so their impact was greater than them just contributing 10% of materiel. Also again, Hitler had a lot of resources tied up in the west because of the allies.

Germany got pretty close to Moscow. A little different of a tactical situation would have been a big deal in that theatre of the war.