Most artists draw them much shorter and rounder than they were in the manga. Actually, as far as SSiB is concerned, Earth Rabbits and Lunar Rabbits aren't very different in ear length or ear shape. The former always seem to have a bend in them somewhere, while the latter are often straighter. Maybe those pins or buttons behind the ears that Lunar Rabbits are depicted with have something to do with it.
Civilian rabbits on the moon have the rounded floppy ears. Military rabbits are the ones with the jagged stiff ears. Reisen II was a mochi-pounding rabbit. Besides all earth rabbits only have black hair.