My Little Pony: Friendship is Magic - Image #240,111

The one on the top was unrefined. The two on the bottom are not only refined, but have fucking ADORABLE emotion animations

i think it's the proportions.
the baby on top has a very large head and a relatively small body.
the Cake Twins have more evenly proportioned features, and like TARINunit9 said, their animations are better, giving them more life.