ACTUAL PROOF:
“On the surface, it was hard to understand, given this pool of manpower, why there should be any manpower problem at all. Why, if Germany could maintain a military establishment of 9,835,000 or 10.9 percent of its population and Britain could support 3,885,000 or 8.2, did American manpower officials insist in late 1942 that 10,500,000 or only 7.8 percent would be the maximum force that the country could sustain without incurring serious dislocation to the American economy? [5] The problem as well as the answer stemmed basically from the fact that the Allies had from the beginning accepted the proposition that the single greatest tangible asset the United States brought to the coalition in World War II was the productive capacity of its industry. From the very beginning, American manpower calculations were closely correlated with the needs of war industry.”
AND
“In January 1943, G-3 warned that the 8,208,000-man Army might approach the limit of manpower available and that adjustments from within would have to be made to secure the kind of Army needed to win the war.”
AND
“In consonance with this economy drive, Marshall approved-in February-a new Army troop basis that called for an enlisted strength of 7,500,000 and between 120 and 125 divisions, for June 1944”
AND
“In late April the committee scaled down its estimates of the ultimate strength from 185 to 155 divisions and accepted an 8,200,000-man total as the planning ceiling figure-the "maximum strength" for the Army imposed by manpower limitations. It recommended that the U.S. Army, and especially the Air Forces, be developed to the maximum strength practicable within the estimated limitations on armed forces and be deployed as quickly as possible”
I would strongly recommend you read the article in its entirety to see that the USA was definitely constrained in its ability to field full strength divisions. Any comparisons to the USSR is ridiculous (and laughable

)