It's funny, my son was absolutely rake skinny until he hit his twenties, at which point - without changing anything about his diet - he suddenly became burly. But he put on maybe a couple of stone and then stopped again, and now he's holding steady at the new, higher weight. It's one of the things that makes me think they're right about weight set points, because there wasn't any change in diet or exercise that caused that, it was just genetic or hormonal. I think our bodies are probably much more mysterious than we give them credit for :)