The Bug:
This has been in the game ever since cats and ocelots were added and was probably intentional back then, but Jappa, the Art Director of the game, has expressed willingness to fix/change this and asked if it was on the bug report site. I mention this just to avoid confusion, as without context this would probably marked as Works as Intended.

Cats and Ocelots share the same model, but cats are smaller, making one of them pixel inconsistent. If I had to guess, looking at it ingame I think cats are the ones that are shrunken. But it could just as easily be ocelots that are expanded; or maybe even both of them are smaller/bigger than whatever the pixels portray.

Steps to reproduce:
1. Summon a cat and ocelot next to each other, ideally with no AI.
2. Compare them; they share the same model but they are different sizes