In the long run the Russians just had too many and the Germans too little too late. Russian tank production was faster than the Germans could destroy them, the Russians just had too many soldiers.
In the beginning of operation Barbarossa, Germany encircled and captured entire divisions, at points 80.000 and 128.000 troops at once. Sounds good? Yeah it SOUNDS good, but they weren't even strategic victories... those kind of numbers meant next to nothing to the Russians.
In 1941 the Germans had close to 4 million (3.767 million) people in the eastern theatre againstthe Russian army at its weakest point. The Russians had almost 3 million people then. For a viable attack - defender ration it is commonly spoken of a 3:1 ratio. The Germans didn't have close to that. (To compare, after day one during Operation Overlord, the allies had a 3:1 to 5:1 and later 10:1 superiority in Normandy). keep in mind that the Russians still had 14 million reservists at that point (former conscripts, so already had military training).
In 1943 the Germans were up to 3.9 million. The Russians by then had 6.7 million and some 10 milion more reserve. Was manpower any problem? Nah, it was more of a problem to equip all those men. Germany never stood a chance. Not with the depth of the Russian country.