Oh there is so many....
Ex-athlete adapting to a more normal life and growing big from it,
Fat lover gets supported and reassured by thinner SO,
Fantasy race experiences modern food and gets addicted to it, etc!
Also for sizes, all sizes are great but I have a soft spot for that small range of weight where a person cannot get off stuff without help, but is not immoble and can still waddle even if it takes a whole lot longer to get to the kitchen!