[Gallagher]

Cho you made me look at stuff for this!?

Well. It was worth it.

To answer your question though, goopers get better at holding their shape when they practice a lot and get enough food and sleep. A good gooper that has been around for a few months can be pretty solid at doggie or kid size. Jello is adult size and an overeater, but they practice holding their shape a lot, so they are more wiggle jiggle slime than mucus slime. Goober doesn't care what he looks like, he just likes eating, so everything gets slimed.