Re: you're an idiot....
If you eat more food than your body needs, you will store it as fat. So some people's metabolism is more efficient at extracting calories from food: those people just have to eat less.
Great, now tell that to the person whose correct calorific intake is 500 calories less than their brain/stomach reckons they need to avoid starvation. Tell them it's simple, they just have to be permanently hungry to avoid putting on weight.
He may be the idiot, but you're the jerk.