Thank you for the positive feedback! 

Anything larger than you (by a small margin) should be able to eat you. Spiky enemies, when their spikes are out, hurt you. This doesn't decrease any HP bar, but reduces your size. With some upgrades, you can do the same to enemies. 

Yes, unfortunately it seems to end at the rocket stage. There was supposed to be an Earth you could eat at the end, but there's a bug that prevents it from appearing.