The population figure is not bogus, it is genuine. If it doesn't tell you what you want to hear that's a different matter. The guys making the pros in any sport were in that 20 - 29 male bracket. Saying the pros came from the colleges does not change the fact that they were also in the age and sex bracket they were in. And there were only 13 million people in that bracket in 1950, 21 million in it in 2005.
I pointed out the skill positions, WR, RB, DB - you are arguing with yourself. I didn't say that many 200+ lbs guys chose it THEN, I was pointing out the relative size of baseball and football players today and how they relate. The relationship between them shouldn't have changed much relative to one another. So unless someone shows me otherwise I have to believe that there wasn't a lot of size difference between the positions I mentioned in football and baseball players. Yes, if you were very small you could still theoretically play baseball and not football, but an outfielder or other power hitter is likely as big as a WR, RB or DB in football. And I would guess the same held true back then. For instance, Willy Mays played his first game in 1951 - he was 5' 11" and weighed 180 lbs. Sammy Baugh, who played QB at the same time was 6' 2" and 182 lbs. Ken Carpenter, a pro bowl RB/WR that year was 6' and 195 lbs. Dante Lavelli, RB on that same probowl team was 6' 190 lbs. The size difference between those probowl football players and Willy Mays simply isn't that great. Sorry I didn't look up other players, those were the ones I could immediately find heights and weights for.