I read this story in the 1990s sometime. I remember it being novel-length, but short novel length, maybe 200 pages or so. The cover art I remember was the most 80s thing you've ever seen - a girl with big poofy hair, shoulder pads, and the kind of VHS-style lighting you only still see in soap operas.
The main character was a girl, American, sixteen years old and fat (her word). She was in a relationship with a man twice her age (I seem to remember he was 32 years old exactly). The book may have even been a sequel - I seem to remember they already knew each other when it started, that we didn't really get a "meet cute". During the course of the novel they move in together, and I believe she gets a job. I don't think he does - I seem to remember him being the "starving artist" type, as his immaturity, and the girl's realization of it, is a significant plot point. I don't remember if they stay together or not at the end of the novel. I do remember that her parents very much don't approve of the relationship and are always trying to talk her out of it. (I don't remember why they don't just forbid her to see him.)
The story is somewhat of a romance. Their relationship is sexual, and I distinctly remember her complaining to him once that he wants too much sex and it's exhausting her. I don't remember the prose being explicit at all though.
I also remember how she either read a book or went to a seminar about why average-sized men like heavy girls, and the speaker/writer explicitly made it an Oedipal thing, that men like fat because it reminds them of their mothers.
I was reminded of this story a little while back and have not been able to remember what the title was or who wrote it, so I'm trying to figure that out.