I think most professional athletes are highly intelligent, but some never learned to apply it to anything other than sport.
Watch Kobe or LeBron on the court. There is an incredible awareness of the shifting context of the game and the available moves. In the greatest athletes it is faster and more fluid than in most athletes.
It's a constantly shifting analytical mode that leans heavily on training and instinct, but is full of novelty. Consider that opponents spend weeks watching tape in hopes of understanding how to anticipate what Kobe will do.
In terms of physical metrics (size, speed, etc.) professional athletes are pretty close to average. I think it is a combination of intellect and general desire to win that makes some people become great athletes.
Because of what I like to call Cartesian Blindness (viewing the world through the mind/body duality) we assume it's simply a matter of great athletes having superior muscle fibers, etc.