What was most decisive was lend-lease. Lend lease enabled the Soviet Union, despite fewer resources (Germans had more steel, minerals etc… with the exception of crude oil), could manage to outproduce the Germans in tanks, planes and particularly artillery. Especially in the years that mattered, 1942 and 1943.
The manpower pool of Germany and its allies, and considering Germany controlled large areas of the western Soviet Union pretty quickly, was no smaller than that of the Soviets. Of course, the Soviets had a one front war.
But it was military production, the quantity of tanks, planes and artillery (coupled with the fact that designs like the T-34 and the Il-2 Sturmovik were pretty decent to say the least) made the difference. Germany was low on tanks by the end of 1942 and also on planes and particularly on artillery. This is why Kursk failed so dramatically.
Lend lease contributed to the disparity between German and Soviet production of tanks, planes and artillery. Lend lease provided jeeps, trucks, aviation fuel, telegraph lines, locomotives, rolling stock, rails, canned spam, army boots etc… so the Soviets could focus on tanks and planes. Richard Overy’s “Russia’s war” mentions that even Stalin admitted in private that lend lease was decisive.