Purely speculating, but it sounds like he's clinically depressed, and weight gain is often a byproduct of depression. Depression is considered a disability at the federal level, so he's potentially part of a protected class.

I personally don't buy the "he's big and tall and it's distracting" argument. From what I can tell, this wasn't part of a film set or the like; it's just a normal party. Since he's a server, what about his appearance is stopping him from doing his job? If appearance matters so much, what's stopping his employer for providing a new outfit? Companies are legally required to provide accommodations in this case if the server has a disability, which might be the case.

From the few excerpts I've read, this just seems like a few judgemental execs didn't want someone to get paid because of his weight, and I have 0 sympathy for that BS. The only other take I've seen is that due to space constraints, having him would be an occupational hazard, which is fair.