Believe it or not, fat people know we’re fat. It isn’t polite to talk about in the real world, but when you’re a professional character actor (an actor who specializes in usually over-the-top or stereotyped performances, be it “wacky neighbor”, “stern police chief” or “fat and ugly woman”) you include that infomation on your resume, and you agent looks out for casting calls that would fit your talent, just like any other part for any other actor.
And yeah, they’re pretty blunt in their notices. There’s lots of money per hour put into casting, and no need to spare the feelings of anyone involved. If you’re too fat or too thin or too tall or too pretty or too ugly, they’ll just say “thank you very much,” or “NEXT!” and that’s that. If you have a good agent, she might be able to find out more specifics about why you weren’t picked, but most of the time it’s a mystery.
Acting is not a job to go into if you have thin skin and ego problems, no matter what your size or looks. People agree to it because they want work. It’s not only thin and pretty people who want to act.