Fat people, by default, seem to be less legitimate than non fat people. Our brains simply look at being fat as a self-inflicted disability, which, of course, it is. Some people even try to frame it as a disease, which, of course, it isn't. Imagine if someone said, "Oh crap, I've got cancer. Better stop eating so much mayonnaise so I can, you know, cure this cancer." If being fat is a disease, it is the most infinitely curable disease ever.
Delegitimizing fat people happens on such a deep subconscious level that most of us don't even realize what's happening. When people look at someone fat, even if they themselves are fat, they immediately mentally categorize them into the "that fat-ass doesn't give a shit" section of their brains. Aside from eating a pound of bacon, nothing makes a fat person feel better than seeing someone else who's even fatter than they are.
Imagine that you're interviewing people for a job. The fat dude on the left should help you visualize the situation...
Now, imagine that this fat guy walks in and sits down. You both wait anxiously while his chair creaks and groans under his girth, but holds. You sigh, examine his resume, and discover that he's totally qualified for the job. Now, unfortunately for him, there will be a part of you that screams, "If this guy doesn't give a shit about those 100 extra pounds on his chin, is he really going to give a shit about this job?" (This applies doubly if the job is "Donut Security Guard.")
The answer, of course, is no.