the most credit for beating back Nazi Germany in Russia was old man winter.....Stalin could only keep pulling back until the Nazis could not go any farther... :^)
And the sheer numbers of soldiers, it's like a German General said during the early days of Barbarossa, it was a fight between an elephant and ants, the elephant may kill millions of ants, but in the end the ants will overcome the elephant and eat him down to the bone.