I’ve found a manga series that has a Weight Gain is call Kenja Chan Wa Satottenai! Is Chapter 6, 13, & 14 in Japanese.
Raised as a transcendental boxed daughter, Yurufuwa is a wise man who has the greatest strength but has no living power.
Transcendence Because that was brought up as Hakoiri Musume, the strongest MAX Yet life force zero of Yurufuwa women's strength wise man-chan.
A fantasy comedy with the disciple who was supposed to take care of his surroundings and everyone in awkward ☆
Fantasy comedy of the disciple-kun and daring everyone who was supposed to take care of their personal belongings ☆
Chapter 6 in Japanese has a female belly stuffed.
https://www.dm5.com/m691827/#ipg14
Chapter 13 in Japanese has a some of male weight gain.
https://www.dm5.com/m823150/#ipg7
Chapter 14 in Japanese has a male weight gain, weight loss, & growth.
https://www.dm5.com/m832923-p10/#ipg1