Another thing about American athletics along the same lines of the "pool" of people, is that a good 95% of our pool of athletes gets sucked into football. Our best athletes are in the NFL.
It goes all the way back to youth sports. Sure we have other sports, but all of the money is put into our football programs. If you were the star soccer player in 3rd grade, you're probably going to be recruited to play RB/WR on the football team. Or if you're a track star, or a pitcher, they're going to hound you to play football.
The difference between that pool and the global pool is that American football is just a more physical game. Soccer requires speed, endurance, agility, and skill. It's more advantageous to be smaller and quicker. A soccer team is full of Wes Welkers. Wes Welker is a great athlete, but i'm not going to call him a better athlete than Megatron.
Even if it's as high as 95%, the US has a population roughly of 300 million.
Which leaves the rest of the world, which is about 96% of global population.
Pretty sure that 96% has pretty good athletes too...