Genetic predisposition for the following:
1. Longer than normal legs compared to torso
2. Lack of muscle.
3. Naturally high Vo2max.
4. For women: Hips more narrow than normal.
Not everyone can be 6'0" 140lbs. Some people are just naturally more muscular regardless of what they eat or how much they run. Also some people have longer than normal legs and a thin short torso while others will have a wider torso. Vo2max is related to weight, the lighter you are the high your Vo2max is. Mitochondira, capillaries, heart size, are all increased through training.
Alan