Gen. George C. Marshall drew up plans to fit out an army of 300 divisions. However, manpower demands of the Navy and Air Corps cut into that. Probably more importantly, we decided to not mobilize all of our military manpower, and instead concentrated on keeping people in industry making weapons for us and our allies. And that became our part of the triad for allied victory: British science, American production, and Soviet blood.
IIRC the Soviets lost more people just at Stalingrad than all the other allies lost everywhere.