Why do most boys eat so much and never get fat?

Most boys in my school eat like pigs all the time and its so annoying that they never get fat at all, even the ones that dont have muscles, my brother doesnt work out but he has a flat stomach (he only runs track) but he has like 3 sandwhiches for lunch and soda and huuuuuuuge dinners everyday and always has ice cream and pizza and donuts while im stuck with stupid salads and vegtables and fruits and grilled chicken :/ i excersise as much as him but am fatter, why?!? like it annoys me SO MUCH