Indeed if one eye is significantly out further than normal, that’s concerning. This happened to Lahi and it was a retrobulbar abscess.
One good way to tell if an eye is doing something funky is to look for the conjunctivia (that pink bit showing at the corner of Lahi’s left eye in the above pictures), which is a structure of the eye that is normally hidden.
Now that said, if you notice two eyes look different, it could be that the eye that’s further out is protruding... it could also be that the other eye is receding. This happens occasionally with my buns, and the conjunctivia ALSO shows when this happens. I panicked the first couple times when this happened, since the conjunctivia and mismatched eyes was the first real sign that Lahi had developed the retrobulbar abscess...
But apparently, according to my vet, “sucking in their eyes” is a totally normal thing for rabbits to do if something is irritating their eyes. It could be something that could clear up with a bit of antibiotics, it could be just debris has been irritating them, but when a rabbit’s eye is sunken in more than normal, rather than protruding more than normal, I’ve been assured it’s not anything truly concerning.
Side note on the conjunctivia: both of Delilah’s always show a bit, the vet calls them “prolapsed”, which means a part of the body that should be internal has become external.
This mostly disappears when she’s relaxed and gets worse when she’s stressed, positively or negatively. It does likely indicate a problem, lops are pretty prone to eye and tooth problems, and she’s already got a blocked tear duct. But no vet has been able to find anything in particular causing it, yet.