Guys. Read the chapter again.
Sting is his normal weight in Minerva’s flashback of the festival, which took place literally days before the events of the chapter. It’s a standard shonen gag for characters to get fat after eating a ton of food. Sting was gonna slim down again anyway, might as well do it with an already established technique.
Also, that kind of gets me thinking… Riseley’s version of Gravity Change forces the slimdown, so does she just use it again to regain her chubby form?