Gosh... I sure would like to know that myself. I think part of it is because in Omega Ruby and Alpha Sapphire there is a Hex Maniac that sells milk so artists see that as an excuse to draw her with giant tits. When you've already drawn a character with giant tits then you might as well drawn a giant ass while you're at it.
The other part of it might just be because artists like that and so they just throw those features on her because they feel like it.