well, first of all, Mei hasn't really been conceived as chubby. fuller in body yes, but not entirely chubby. secondly, that is an actual problem some character models have with their rigging, it happened in Xenoblade 2 when certain characters were hugging up against walls. it's probably the character model sticking hard against the framing of the character that causes their stomachs to jut forward like that. you can tell Mei's stomach is also much further out closer to where her shirt ends, there's a curve closer to the pants that looks completely unnatural to the character model, almost like a bad Photoshop effect.Mei is just proof Blizzard has no idea how to actually model a lady that's not skinny and just snaps the model's spine and covers them in heavy clothes to give the illusion they "might" be chubby
Her model and rigging straight up need a complete overhaul, and Blizzard can't complain when they were the cowards who decided to make a chubby girl that wasn't actually chubby :p