Shockingly, I'm not sure they do! The only television show example I can think of is Drop Dead Diva, and the entire central plot of that show is that she's actually a dead model in the body of a fat lawyer....which means we're allowed to like her specifically because her fatness is not her fault. And aside from Hairspray, the only movie I can think of is Britney Runs a Marathon, in which case she starts sleeping with the guy after she starts losing weight.

Oh! Except for any movie starring Queen Latifah, and that one movie Mo'Nique was in years back.

Otherwise, nope.

Even the star of My Big Fat Greek Wedding eventually lost a lot of weight in hopes of making it in Hollywood.