In no particular order
1. Character is weight conscious but a food addict. Gets magic/high tech gubbins that keep them skinny. Eats like fuckin crazy. Thing keeping em skinny breaks and they become super obese.
2. Character gets into a position of authority and they get rapidly corrupted by being in a position of power. Uses it to eat whatever they want and gets really obese.
3. Character loves being fat and thinks they're super hot while being fat. Actively gains more and more while still showing off to break societal norms of skinny = hot and replace with fat = hot.