The reason why Soviet production is so low in 1942 and 1943 is that 30-40% of their population was under German occupation. Thousands of factories and mines had either been overrun or destroyed.
But the Soviets managed to get by because they of the Lend-lease raw materials. Lend-lease provided 55% of aluminum, 45% of copper, 40% of lead, 28% of tin, 52% of tungsten and 73% of molybdenum.
Also, the Germans never fully utilized their manufacturing capacity for war production until 1944.
Truman's idea of backing one side when it was weak and then backing the other when it became weaker wasn't really in the cards. Democratic countries have to make wars into moral crusades and can't shift gears as fast as dictatorships can. Strange as it may seem there's still something like honor or principle that prevents countries with representative governments from switching sides. It does happen on a small scale, when people aren't paying attention, but it's harder to pull off a major reversal of policy when the whole country and world are watching.
Germany was like the swordsman in the Indiana Jones movie: Americans just got sick and tired of them and didn't want to put up with them any longer, so we weren't going to bother with any attempts to court their favor (maybe that's not the right metaphor but it does capture something). During the war we just wanted to get them off the scene as a world power and international threat. Once that happened, we'd figure out what to do next, rather than playing some complicated strategic balancing game with unpredictable outcomes.