Why don't I get fat?

At the moment I’m fasting, and it has made me realize that I eat a lot normally. However (and I’m happy with that, of course), I never get fat. In fact, I’m pretty underweight if you ask me. I do play football a lot and cycle to school, and when I eat a lot it’s mostly dinner, not sugar-rich in-betweens. However, surely if you eat a lot then it doesn’t really matter what it is?