I mean, it's simply a cartoon after all, right Anime Flavor? It's not like as if the writers introduced any real depth, emotion, reality checks, or situations that can be seen as 'dark' right? … Since it's not an anime, clearly any development in a kid's show can't be done-

… … … Oh.

I think Steven's a pretty good design as far as "child" goes. Considering how varied every character is, and how large Greg and Rose are, it makes sense. Plus, he's just adorable as sin! How can not look at him and immediately want to hug him?

And it's not like his pudgyness is that important to the show. Other than a single gag with his gut and the transporter (at the start of the series) they've not done anything with it.