Extreme obesity

How about Bed: A Novel by David Whitehouse?

welllll there is a charcter in the house of cards series by the same dude who did game of thrones..that gains weight by kinetic energy. worth looking into. sorry wild cards series. did I mention later on she absorbs a nuclear bomb and ends up huge?

also there is a little hot scene in horns by joe hill.

Don't suppose you know where I could get a copy, do you?

There was another Goosebumps book called "Don't Fall Asleep" where this kid is told never to sleep in one of the rooms of his house. So, naturally that's all he thinks about doing. One day he does and he just starts spinning through alternate realities. In one of them he wakes up as a fat kid.

Maybe RL Stein was a closet FA

Burl, one of the skill coterie villains in the Assassins trilogy by Robin Hobb starts off as totally built and then spectacularly runs to fat.

In the third book in the teen fiction series Keys to the Kingdom by Garth Nix, Lady Wednesday (now known as Drowned Wednesday) was royalty at some point, but then got cursed. In the book she's a whale that's hundreds of miles long, but in one scene she uses magic to transform back into a human to meet the main character briefly on an old ship.

They meet in a dining room and as they speak she has to be constantly eating, quickly gaining a ton of weight until she sinks the ship.