I've got a pretty strange and confusing case of manga weight gain.
Kyoko Houin from Kodomo No Jikan. The gain begins in volume 6 page 81. She attempts a diet at first, but quickly abandons it for plot reasons and ends up getting even chubbier between the pages 5 and 36 of volume 7. She then resumes the diet as a New Year's resolution and even begins to exercise more vigorously, but by the time she undresses herself on the page 107 of volume 7, she still seems to be just as, if not more pudgy than she on the page 36 volume 7. From the page 109 volume 7 and up to the page of 109 volume 8 things are still rather clear; the weight gain seems to stick, she still eats and drins plenty and there are multiple gags and mentions related to her belly, acknowledging that she still hasn't lost weight. There are even some very obvious shots of her belly on the pages 162 volume 7 and 54 volume 8.
From this point onward, the things get way more bizzare and uncertain. On page 109 volume 8 it's heavily implied that her potbelly is still an issue, since she even gets mistaken for being pregnant by the main protagonist's mother. She spends a few days there, while still drinking beer and eating without restrictions to the point of giving herself a hangover. However, than we get a shot of her waistline on page 135 volume 8 and it seems suspiciously narrow, almost the same way it was before her weight gain began. You could dismiss this as the clothes hiding her belly, but here's the kicker - on the very next page (136) she gets fully naked and her stomach is nothing like what you see on the pages 36 and 107 of volume 7, it's way more similar to the very first shot of her bare stomach on page 43 volume 2. Despite the implication that her belly has remained the same size since page 107 volume 7 and the fact that she has not changed her eating habits at all, it's obviously not the same potbelly we got blessed with.
From here on out, it's just a downward spiral. Just like earlier, there are no implications of her exercising more often, and yet by the time we reach page 62 volume 9, we get a brief glimpse of her midriff again, which once again looks nothing like the reveal in volume 7. We get another reminder that she has not stopped being a glutton on pages 110 and 112 of volume 9 and fears getting "even fatter", possibly meaning that she still hasn't lost any weight. There's even a shot of her stomach on page 113 volume 9 that kind of resembles the potbelly from volume 7 and the fact that her old panties don't fit as well as they used to. From this point onward and up until the very end, all of the shots of her midriff that we get, whether bare or clothed, make it look like she's lost pounds and returned to her initial weight.
Did the weight gain stick, or did she manage to lose weight towards the end? It might just be the art inconsistency, which is supported by the fact that even during the 100% confirmed weight gain and chubbiness between volumes 6 and 7, certain panels still depicted her as just as thin as in the beginning of the manga with her wearing the same clothes, so did the artists exaggerate her potbelly on the pages 107-109 volume 7 for dramatic effect, meaning that her stomach was never like that and has always looked like what we see on page 113 volume 9 ever since she started to gain weight? It's somewhat possible that she's lost a small part of what she's gained, not all of it by the time she arrived to the protagonist's mother's house, but I find it unlikely given the aforementioned inconsistencies of the portrayal of her figure after the weight gain, her talking about her potbelly right before the trip to the house in a way that signifies that no weight loss occured during that period at all, her getting mistaken for being pregnant when she arrives there and her never trying to change her diet in any way before, during or after volume 9. Does it mean that the weight gain actually sticks to the very end and that the artists just failed to keep her figure consistent, forgetting to add any fat to her naked or clothed stomach shots after volume 9? Have the creators ever clarified any of this on some sort of a blog or during an interview? I'm genuinely baffled by this.
I hope that this autistic drivel didn't turn out to be too incoherent and that someone way more knowledgeable on the subject of this manga and its creation can actually explain whether she keeps the weight to the very end or not.
Now I shall provide the link to the manga in its entirety in case you want to come to your own conclusions, as well as all the pages I've mentioned during the rant with this post and the subsequent ones.