So it's a gag manga turned anime about a psychic high schooler who can't stand people and is surrounded by nut job cliches
Anyways, they have 2 episodes with weight gain on a character named Chiyo.
https://www3.gogoanime.se/saiki-kusuo-n ... episode-24
This is an English sub from the first season and here's the dub
https://www3.gogoanime.se/saiki-kusuo-n ... episode-24
https://www2.gogoanimes.tv/saiki-kusuo- ... episode-24
The is an English sub from the second season that just finished this week. Chiyo's weight gain is less focused on this episode, but it's present.
The thing about the sub and dub is Chiyo's voice. It's a great dub, one of my favorites, but Chiyo doesn't have a fat girl husky voice in the dub.