That is a tough question. I would say the body type that really eats like they want to gain weight. Eating is the biggest problem. Many kids think they eat a lot. They really dont. Whenever they tell me they cant gain weight I ask them the following questions.
Me:Eat more food
Them: I eat all the time and still cant gain weight.
Me:What did you have for breakfast?
Them:Nothing, but last week I ate 4 double cheeseburgers at mcdonalds.
You will gain weight if you consistently eat more calories than you burn. That means eating all day every day.
Probably though to better answer your question, it is the big skill body types in football. LB's, FB's, TE's, etc. I don't know why but that has been my experience.