It accounts for some, but I think the MOST common is have-zero- self-control-and-stuff-your-fat-face- itis. You should ask yourself why you *want* to think ill of fat people.
Apart from citing any specific example, generalizing such as you have can be explained by nothing beside simple malice.
I'd don't think ill of fat persons, in that I have fat friends and hold nothing personal against fat individuals. But I think ill people like this anchor who try to say that those who condemn fatness are the bad guys (again, I'm not suggesting government has a right to "fat-control.") In many/most cases fatness is entirely preventable by some pretty simply steps. Fatness is not a disease and fat people are not victims. I think ill of those who suggest fat people are victims and should be a protected class.
And I'm not going to pretend fatness is healthy, advisable, or attractive. And I will crack fat-jokes.