I don't really care about "fatphobia" in media, and I don't think this instance is like, bad, but I have something to say about Mizisua, body types, and how I think Mizi's design is flawed
I'll preface this by saying I don't *blame* vivimeng/anyone else that had a hand in designing the cast. I don't think this is a moral failing, but an artistic flaw. Art is meant to communicate things, not just be pretty to look at.
Mizi and Sua both have a lot of their life dictated by their body type, with Mizi being sexualised and objectified while Sua is infantilised. Of course their bodies are not all that contribute to this, and I'll elaborate on Mizi's in a bit, but for Sua her infantilisation is supported by her low mental strength. Mizi doesn't have to *physically* overpower Sua to have power over her, but the comics repeatedly show us that she *can* and that she's aware and enjoys this fact. I think Sua captures this perfectly in her design and her outfits too. She looks delicate, she moves that way in the animation, etc.. Mizi on the other hand, by virtue of being drawn really similar to her (with barely any body difference besides breast size), does not fulfill her design role.
Mizi's objectification by the boys around her in Anakt garden and of course their alien audience is a key factor in how she views herself. It's where she gets her "true face" persona. She feels guilty for the body she has and how she is perceived because of having it, so it is a conscious decision to depict Mizi's view of her own "true" (read, evil) face as attractive, and you can tell from the art that they *do* intentionally sexualise her in these panels.
The one thing that sticks out to me however is that no matter how often Alien Stage tries to tell us that Mizi is uniquely attractive, they don't dare to have a character that looks too different from what they view as pretty, and so they never make Mizi look too different from Sua. This muddies the contrast mizisua have. From the way Sua talks to Mizi in The True Face, we're meant to understand that they have very different lives even though they're both pet human girls in Anakt, but without the visual support of them looking different, this dialogue reads as though the only reason for this is because their guardians have different ways of managing them. We fail at show-don't-tell, we constantly *talk* about how attractive Mizi is and how different she is to Sua, but the reluctance to draw anything resembling fat (such as curves on a woman!) obscures this.
And the thing is, I can tell they tried!
You can usually tell that they *did intend* to make it obvious that Mizi is physically bigger than Sua. She's a little taller(!). I just think that despite the effort, their bias for what they consider pretty overshadowed this intention.
In conclusion: Mizi shouldn't look as skinny as she does if we want to convey that her looks are the reason she's sexualised and objectified. A curvy/femininely attractive woman inevitably would have curves.