I don't think there's a fat-hate "movement" tbh. I think it's always been popular to make fun of fat people. Harry Potter and the Philosopher's Stone had a fat child be described as neglected, spoiled, and a bully, and therefore it was justified to magically force the child to grow pig parts that had to be surgically moved. Published 1997.

I guess somewhat more recently we have people like Lizzo trying to push back on things like this, but the CDC has recently pushed a recommendation for bariatric surgery on children as young as 14 so. Shrug, I guess.