There should be a list of books about fat girls. Maybe there is.
Here is my very short one:
She's Come Undone by Wally Lamb
Eleanor Rigby by Douglas Coupland
Nothing's Fair in Fifth Grade by Barthe DeClements
The last one has a fat girl but she's not the narrator or protagonist. The other two star women and their weight is a part of their indentity and part of the book. Here is the weird thing: both those books are written by men. They seem to hit the mark, though. I wonder what kind of research they did.
I am in search of a book about a fat girl or woman written by a woman.