What you are saying might be true for something like magazine covers, but that isn't solely a female issue. It's not like men's health magazines don't feature impossible bodies only achievable through roids and other supplements. Just seems like almost a non-issue.
I don't really see the same issue in cartoons and I think Glimmer is really just a token for a certain demographic or the staffs own personal delusions about making some fat girls day because a cartoon character is fat. It doesn't really detract from the story, so thats why im fine with her overall. But the idea that her character is somehow progressive or important because of her body size is just silly. Just seems like a lazy way to pander to a very small nice demographic without putting in any real effort or message. Sure, it's ok to have a little more weight. But you can still do different body types without going full fatty just to be "le transgressive XD." Because it's not transgressive at all. It's just a token character that is drawn fat 'because we can'. It's a kids show, so who really cares and all etc etc. But personally im okay with my fantasy being fantasy. Art shouldn't have to reflect every niche aspect of society, especially in kids shows.
The show is good so again, it's just a nitpick. But when I see people overreact and praise a character solely for it's body type or sexual orientation, I feel like that is just sad. Let's be real, Glimmer isn't there for the kids. 99% of kids wont give it a second thought, they are just watching cartoons. It's there for the late 20s mid 30 something fatty who wants to prove a point about some body image narrative she has in her head. Fat people are not an oppressed class and Glimmer wont make their life better. I bet the designer sure felt good about herself when designing the character though. Like she was doing something real brave (its not). Just a hunch.