Here's a few of things...
Genetics due play a huge role. Some people can eat 5000 calories a day and not gain anything, other people can eat 3K and gain weight like crazy. Look at Dexter Jackson, I don't think it's possible for Dexter to get fat.
Generally fat people lack will power; however, I know a guy (5'7" 170lbs with a belly) who calls his wife a discussing cow to her face. He claims she doesn't have will power, but the reality is it is very hard for her to lose weight. This guy has a lot less will power than his wife does, he has never dieted in his life, he eats junk food everyday all day. The difference is their genetics.
My point is that MOST people (skinny and fat) lack will power, people getting fatter has more to do with genetics, food addition, high fructose corn syrup, and availablity of food.
Bodybuilders simply possess a much higher level of will power than the general population.