Skinny people see being fat as a moral failure and thus losing weight through any means other than starving yourself and exercising 24/7 is unacceptable.
Meanwhile, most skinny people just eat until their full and aren't hitting the gym, it's just their default.
But, on the other hand, most fat people have lost and regained weight tons of times, so when they see someone losing weight without starving themselves and hitting the gym, they get jealous.
That's the bit people don't understand. Fat people on diets are starving and constantly uncomfortable. It's a 24/7 attack on willpower (and it doesn't magically go away when you hit goal weight). The weight will almost certainly come back, because willpower is a finite thing, and it's more and more demoralizing each time.
Skinny people don't have to think about it.
When fat people can lose weight without thinking about it, everyone else feels threatened.
It's the same reason everything else sucks. Literally everything needs to be gatekept.
Why can't education be cheaper, or loans forgiven? Because they had to suffer, and so do you.
How would it possibly be fair to somebody who died of cancer because they couldn't afford care and then someone else just gets care for free? That's not fair! They gotta pay!
Why can't we have access to abortions? Because some other dude was responsible for an unexpected pregnancy and got stuck in a loveless marriage raising a bastard they didn't want, and now you do too.
Nothing can be easier or improved as long as other people had to have it worse.
Same shit, different story. "I did it so you have to have it as bad or worse". It's jealousy, all the way down. I blame the church.