“there isn't a law of physics” all right, but for evolutionary reasons I'd expect that for all except a non-sizeable fraction of people, the minimum weight below which they would die from starvation (or even the minimum weight below which they would stop being fertile) would be way below the maximum weight above which their obesity would turn potential sexual partners off. (And if this isn't the case, that would mean that today's fucked-up beauty standards --mostly due to the preponderance of very skinny models in the media IMO-- are by far even more fucked up than I thought.)