Actually this one is pretty old, and I think it stems from the idea of wealthy people being fat and selfish. I'm thinking of some of the old political cartoons of the Gilded Age ,where wealthy men were shown with big round stomachs, and (for some odd reason), large mustaches. For all we know, this association is probably a lot older, going as far back as the Medieval Era, the only difference now is that instead of being applied to the wealthy, who are now seen as examples of food abstinence, its a stereotype being applied to every other socio-economic class.
My big issue with fatphobia in movies is that fatness is often used as an example of how a character's life is "out of their control", and sloppy. I became aware of this while watching one of my favorite comedies, Death Becomes Her, where Goldie Hawn's character is shown in a fat suit, wearing no makeup, (presumably), sitting in a house overrun with cats, stuffing her face with something out of a jar, while using her bare hands. Someone is furiously knocking on her front door because her rent is due. Her life has spiraled completely out of control. One way that she is shown as having regained control over her life, is the next time we see her, her hair and makeup are impeccable, and she is slim and svelte, in a slinky dress.
And this is not the only time that Hollywood has equated having a sloppy, out of control lifestyle with being fat. I hadn't connected the stereotype of selfishness, and meanness to it, so what appears to be happening here is that Hollywood is laying all of its worst character traits onto the bodies of fat people.
Thank you for writing this. Its given me something to think about more deeply.