Okay, I'm not being mean when I say this Aeoschan but your question just seemed kind of silly. It's like going to a gallery that features an artist who specializes in large breasts and asking, "why do all the girls have big boobs?" The simple answer is that it's just the way the artist wanted the character to be designed. Tsuji Santa designed Pochaco to be a bit chubby, and that's it.