I think an interesting idea for a girl is we find her losing weight so we see a few stages where she actually gets smaller, but she does this not to be thin but so she can pig out for a few months at a time. Then while we help her pig out we convince her to get even fatter and keep convincing her that she should go bigger before going back on a diet to start over.
A girl who loves getting stuffed but knows how to lose the weight so she can keep stuffing herself without becoming immobile.