I think the unstated assumption OP is making is that fat people got hooked on food, and can't be responsible and give up their addiction. And while it may be hard to do, you only owe your inability to stick with quitting for your continued problems. Born again skinny people are particularly severe in this regard, since they've likely overcome their own personal issues with food (for now, at least) and so see that experience in everyone else. Maybe they made physiological excuses before, and tend to discount those issues in others; maybe their own feelings of weakness don't allow them to be especially empathetic.

And, the analogy to smoking is fatally flawed. Nobody needs cigarettes, but everyone has to eat.