Genetics follows money and soon takes over. In 1900 the average professional MLB pitcher was 5'11" and 175 lbs. In 2010, the average pitcher was 6'3" and 210 lbs. Average size of NFL player in 1920's was 5'11" and 198lbs. By the 2000's the size was 6'2" and 302lbs! In the NBA of the 1930's the average NBA player was 6'3" but by 2000 the average player was 6'7". What is more, in 1940 barely more than 1% of the players in the NBA were over 7'; but by the 1960's nearly 12% of the players were over 7'. Sure, the average size of an American has risen over these same decades; but not by this much. This is not natural selection, it is artificial selection.
So that skinny kid at your BJJ academy, who wrestles in school, and boxes at the community center. That kid who is too short for basketball scouts, too skinny for football scouts, and who lives in the wrong neighborhood for baseball; that kid might be a genetic rarity. If he can keep his length without gaining too much mass (think chicken-legs); if he has quick-twitch explosive muscle in the right areas and slow-twitch endurance muscle in the right areas; if his lung metabolism is right; and if his mind and sense of self are strong. It might be that the whole mixed martial art community is looking for that kid.
That kid might one day be the champion of the world.