OK, I know there are a few people who either overeat massively or starve, but I think that in general if people are 'heavily built' or 'lightly built' that is just how they are. I would say most of my friends eat roughly the same amount, yet there is quite a big difference in size between us. Some people are just naturally slim and maintain it without effort, and others are naturally bigger and it would take a massive effort for them to maintain a size 8/10 whatever.
It's a massively unscientific theory, I know, mainly based on my personal acquaintance!