I just googled Population of XXX in 1940. Not saying the numbers are 100% accurate, but they are probably pretty close. Well, the US Population I know is accurate since I got it from the United States Census Bureau from 1940.
France/Italy/Germany I am not sure on. But it seems plausible.
China does seem really weak in those numbers…not sure if google was accurate there or not.
Russia is also probably pretty accurate. A lot of Russian soil is uninhabitable in 1940 - cannot farm it, cannot survive temperature extremes, too far from civilization, you name it. I think a lot of your reading here might be left over civilians, most of Germany was involved in the war - unless you literally could not hold a rifle. Even children were pressed into service for the Germans.
Of course, feel free to go find your own numbers. Not overly easy finding accurate ones because you are at the whim of who posts what for numbers, lol. All of my numbers (except the United States of America’s) could be wrong, or dead on…