Although I suppose it's the premise, I'm not thrilled about the physical changes. Thick gyaru is best.
Yeah, I feel like this manga's premise isn't a very sustainable one. A major part of the initial appeal of this manga is that it has a thick/chubby female lead. If she loses weight, then the series loses what makes it unique and just becomes yet another fanservice romcom with a generic thin-yet-busty tsundere. However, if she constantly fails at losing weight, the series would start to feel repetitive and there would be no sense of progress.
Elf-san wa Yaserarenai (Plus Sized Elf) goes the "constantly fails at losing weight" route but it works there since the story has enough wacky magical reverse-isekai shenanigans to stay entertaining despite the lack of progression.
This series, on the otherhand, doesn't have enough going for it to go that route without getting stale but if the female lead actually does get thin then there's no longer a reason to choose this series over all the other fanservice romcoms out there. I think this manga's premise would work better as a oneshot than as a serialized work.