Yeah, making the curse leftover by that semi-eldritch god lizard beast be one that just makes you fat is a direct assault on my suspension of disbelief. So, I'm just going to pretend this didn't happen from now on.
Guys remember that the curse was making her fail at summoning the hero. It kinda makes sense if the rules of the world are very tied to the physical state of the people similar to how in the Potato Elf manga she is not able to go back if she is fat.