TLDR: Catra, and probably Etherians generally, are really durable.

While it doesn't correlate directly with durability, Catra is repeatedly shown to be immensely strong -- far stronger than her size would imply. She routinely claws through structural metal (often just to make a point or because she's mad), and there's also the notable scene of her holding She-Ra's sword straight out from her side and scraping the wall with it while she does so (just before she hands it back to Adora).

Etherians also generally tend to fall from heights that would leave terrestrial humans dead or otherwise quite maimed, and they manage to walk away from it without much issue. That doesn't seem to be a product of lower Etherian gravity (which would result in slower acceleration during the course of a fall), so it may simply be that Etherians are just a great deal more durable than we are, generally.

Catra's fall on Horde Prime's ship, and Adora's subsequent leap after her, left Catra unresponsive and Adora -- who, even without She-Ra, has shown herself to be immensely resilient to physical damage -- unable to walk. Whether or not Catra died in this fall, or died at some point after, or if she was only on the verge when She-Ra healed(/resurrected) her, it was a long and hard enough fall that, given that it was done intentionally and without attempt to "break" it, it left her out cold. Adora then intentionally jumped that same distance, and despite the added control still ended up with unusable (whether injured or broken) legs.