^I think you're asking about Falin, not Marcille. Anyhow, I think it's similar to how fan-artists often draw characters with exaggerated bodies, and it sticks to the point that it becomes the norm for artists and the fandom. Bigger ass, bigger boobs, thicker thighs, and so fourth are common. Then people are surprised when the character in the official art doesn't look like that. With Falin she wears kind of looser / baggy clothes in series, but to my knowledge her stats put her in average weight range, healthy, and in the manga/anime she looks like that as well to me. Not super slim but in the middle. However people enjoy drawing her heavier, and that's how a lot of the fandom sees her, so that's just how a lot of the art is going to roll.
I would like to note as well that I'm not saying how artists should draw her. Personally I enjoy it more when artists draw closer to canon builds on average, but I'm also not going to tell anyone what to do, and this art is fun.