Some vets still hold with the 'anaesthetic is dangerous for smaller animals', I know the vet I used to go to would never have given an anaesthetic to a rabbit, they didn't even know neutering was possible. Anaesthetic advancements are still really quite recent.
Obviously the smaller the animal, the higher the risk will still be, but the risk is much smaller than it was. So I think more rabbit savvy vets maybe wait because they know rabbits will carry on growing, so they might as well wait until their size is at the minimum risk, if that makes sense (it might not, sorry
but I know what I mean!)