>>269992I think I've figured out some more of it through Google translate and DeepL.
It seems like she became a glutton addicted to sweets due to the stress of becoming a school president chairwoman or something to that effect, but that's not all of it. Her best friend, the red-haired girl with pigtails, is a really good cook as well as an owner of a food cart and seems enjoy making food for the pink-haired girl to indulge in, which could be a part of it.
Moreover, it's quite possible that the weight actually sticks; a flashback in episode 6 of the Grindhouse is stated to have occured 2 years prior. The events of the flashback in question are depicted in episode 3 of the Gridnhouse, where the pink-haired girl is still skinny, meaning that it probably took place in before or in between the events of the main story, or even shortly after it ended. Regardless, there is a clear continuity to this whole story and the fact that the Grindhouse episodes 9 and 11 still depict her as chubby makes it even more likely that she gains weight shortly after the main story concludes and then never loses it.
Whatever the case may be, I find it really hot how the stereotypical airhead bimbo with big boobs and a wasp waist is the one who ends up becoming the overfed fatty of the cast. Lastly, my translations indicate that the pink-haired girl's name is something along the lines of Chiharu Misumi.
If you feel like my analysis is wrong in some way, feel free to read and translate more of it yourself.
(the first three pictures include her before she's gained weight for comparison)