Ignorance. Look at the art of the ordinary people, and you will see no fat women there. There is a difference between status and beauty. Also the kind of people who could afford to have themselves painted weren't selected because they were beautiful. They were painted because they paid for the painting.
Rich people ate a lot more because they were greedy, without discipline, and simply didn't care. They were already married, and there was nothing anyone could do about it. Their husbands had affairs with other women. There wasn't a single mistress of Louix XIV who was obese.
The people who are telling other people that fat was once considered beautiful are projecting their own desire to believe this onto other people. And, of course, overweight people are only too keen to believe this.
Again, prosperity, power, and status have nothing to do with beauty. If you really want to claim that, then you have to be okay with only rich fat people being acceptable and not poor ones.
The natural form of humanity is NOT obesity. I've just written an article about body positivity and I supply vintage photographs and art from prior centuries to prove this.
Also, put the words "Greek sculptures of women" in Google images, and all the statues are slim. And everybody has a few creases when they bend over - even when they're very slim.
Do the same thing with the words 'Roman women during the Holy Roman Empire.' Same thing. Or google African crowds in Africa.
Obesity is not beautiful, and it never has been. It is unhealthy, and it is way past time personal feelings of inferiority were not the driving force to get it accepted. Obesity must be eliminated, and it is the government's job to legislate against the causes of it.