Here's a few movies that haven't been mentioned:
Zuckerbaby (1985). A German film directed by Percy Adlon, starring Marianne Sagebrecht as a fat, lonely mortician who develops a crush on a married subway conductor she sees while going to work, and pursues him relentlessly. Its something of an art film, and does not have a happy ending, but Sagebrecht is very good in it. It was remade for network TV, premiering on CBS on Valentine's Day, 1989, as Baby Cakes. This starred Rikki Lake and Craig Sheffer as the smitten mortician and the object of her affection. It is a more conventionally mainstream film, and this one does have a happy ending, which works well and leaves viewers with a smile on their faces. Lake is delightful.
As she was in a movie I can't believe nobody has yet mentioned, Hairspray (1988), John Waters's quirky PG rated comedy, which also starred his muse, Divine. They played, respectively, spunky fat heroine Tracy Turnblad and her mom, Edna. The first film I can think of that had a fat girl as the lead character - and she gets the guy! This was, of course, turned into a Broadway musical in 2002, staring newcomer Marissa Jaret Winotur and Harvey Firestone as Tracy and Edna. This was a huge success, and begat a film version of it's own in 2007, with another newcomer, Nikki Blonsky. I can't comment on this version. I refused to see it, since casting a thinner actor in a fat suit (John Travolta) as Edna Turnblad (in a movie at least in part about size acceptance, and large size empowerment) seems to me equivalent to casting a Caucasian in blackface in one of the African-American roles. Finally, NBC aired a live broadcast of the musical with a new cast in 2016. As with the other versions, a fresh face made her debut as Tracy Turnblad, Maddie Baillo.
In 1987, director Percy Adlon and star Marianne Sagebrecht reteamed for the English language cult film Bagdad Café. It also starred Jack Palance and C.C.H. Pounder. A memorable scene is when Palance's character paints a nude portrait of Sagebrecht.
Another made for television film, this time broadcast on the Lifetime network, was Queen Sized, (2008), starring Nikki Blonsky, from the 2007 version of Hairspray. This is a high school-set dramady about a fat girl who is nominated for homecoming queen as a cruel joke, but goes on to win.
Finally, there is the independent 2002 film Real Women Have Curves, starring American Ferrera. Based on the acclaimed play written by Josefina Lopez, and directed by a woman, Patricia Cardoso, it recounts the struggles of a family of Latina women in East L.A., fighting against racism, sizeism, and constricting expectations. None of the women are super size, particularly Ferrera, but it is an engaging film detailing the lives of characters who are seldom granted the spotlight in movies. It also stars George Lopez, in a supporting role as a mentor/teacher.
As far as the movies that have already been mentioned. . .I fucking hate Shallow Hal and especially Feed (and I'm a horror movie aficionado!). On the other hand, I love City Island. As far as I know, its the first mainstream film to include feederism in it's story (even if it is a subplot). Not only that, they don't treat it in an exploitative or distorted way. Who woulda thunk?