Given the less controllable tail end of the process creates a widely variable amount of flammable solids and gases, it isn't as though it is just the inputs you should be considering. I'm skinny and fairly sedentary and I know damn well that with a different set of genetics and the same behavior, then I would be fat. Also, seems that in terms of lifespan, I may be better off being a bit overweight than slightly under, really, the benefit seems mainly aesthetic for most people, which is a crap reason to guilt-trip a society.