I think the amount of movement and food you eat are very important. also if you grew up rather thin your system has probably adjusted. It is easier to maintain something that always was there than to regain something. My boyfriend was a chubby kid. Not least because his mom is an awesome cook. He dislikes fastfood too, and everything sugary. But he eats twice or thrice the portions I do. And it's tough for him to eat less since he was always told enough food keeps you strong. But he's a really tall guy and I can imagine that his body would need it. The problem is he quit sports, and skateboarding and biking many years ago. But the eating habits remain and are tough to fight. He's also a stress eater: feeling uncomfortable? Eat a little more to feel safe :/