(Just a general warning, the weight gain makes this girl VERY UGLY 😮💨)
The series is "I'm the Saintess's older sister, but after I stopped gathering special magic stones and herbs for my sister, I ended up being happy with a foreign magician!"
The main character's younger sister Eileen is a ruthless but very beautiful brat who is said to be the next "saint", putting her in a very high class society and position. While her lover is away in chapter 6, she begins to spiral and only finds comfort in eating an excessive amount, by the time he returns she's become a bloated ugly pig
The reason the change in her appearance is so drastic is because her sister (whom she cast out) was the one who always took care of her skin and health, without her, Eileen became unkept and unsightly. To my knowledge she keeps the weight for a while, I don't know for how long or if she loses it either
Link to the chapter: https://comix.to/title/rq7jm-im-the-saintesss-older-sister-but-after-i-stopped-gathering-special-magic-stones-and-herbs-for-my-sister-i-ended-up-being-happy-with-a-foreign-magician/9380042-chapter-6
This series is also adapted from a light novel, I'll also include the chapter where she gains weight if anyone is interested, it gives the full context there: https://ncode.syosetu.com/n9580hx/16/