I bet it's residual genetic instinct from a lineage that lived throughout famine. Obesity used to be considered an attractive quality because it meant you were rich and had food security.
Obesity was attractive in the same way $25,000 Cartier glasses are attractive today; of those who can afford frames that expensive, few are going to spend their money on them. It takes a certain kind of rich person to look that absurdly rich, and generally speaking it's seen as obnoxious to idolize a standard like that.
And frankly I think it's stupid to believe there are families that lasted long enough to shape the genetics behind metabolism and also made it to the modern age by the skin of their teeth, only to foster children the size of buses. More of us eat like crap than ever before, and we have our modern standard of living to thank for it. It is so stupidly easy to be out of shape let alone overweight. Any food we want is a car ride away at most. Our lifespans are the longest they've ever been, sometimes spanning longer than a century, and we waste half of them being relatively sedentary.
I'm willing to listen to the excuse of stuffty upbringings and mental illnesses leading to out-of-control obesity, but it is completely implausible that anyone in this world is fat just because great-grandpappy had a slow metabolism.
