Casually Walking:
Technically everyone has it, let me explain
It is widely believed that every single person will develop a want for a bigger partner under stress, because eons ago, when humans didn’t have an abundance of resources, stress usually came with a drought or a bad harvest, which meant someone larger had food
In the sense of the people who’s preference never “turns off”, well, that’s more difficult to gage, someone would have to go around asking everyone if they like fat, being fat, or any other relation to the fetish. And I’m sure a lot of people wouldn’t admit it due to the stigma surrounding beauty
So perhaps fat fetishism is based on stress in childhood