I think a lot of this has to do with cultural differences. In East Asia there is blatant public shaming of people who don’t fit thin body standards. It’s especially bad in South Korea. Look at how people in K pop groups will faint on stage because they don’t eat enough trying to keep themselves thin enough to not get ripped apart. Many trendy clothing brands only have one size because you’re expected to keep yourself within those sizes (although this is less of a struggle because people are much more homogeneous than in Western countries). He comes from a culture without acceptance of plus size bodies, as well as healthier food in general and smaller portion sizes. A regular size fast food meal in Asia is comparable to a kids meal in the US.
It’s possible he’s just a jerk, but it’s also possible that it’s just a cute nickname and Devin doesn’t have an issue with it, she had several plush pigs on display in her room, so maybe she just likes pigs so he called her piggy? It’s hard to tell since we know production directs a lot of these interactions.