Cats are more agile, but a kick from a hefty bunny's back legs could severely injure or even kill a cat. Cats could scratch and bite, but so could a bunny if cornered.
I think its down to the cat being able to get away more easily.
Not sure, while cats are agile rabbits can be very vicious. My friend had a very large white rabbit as a pet and it had the run of the garden. When she came home from shcool she had to run from the garden gate to the back door because the rabbit would run at her and try to bite at her ankles. She also maintains that sometimes he lay in wait for her to come back, hiding in the bushes etc, but I am not sure how true this is!!
A large male rabbit will give an average cat a good fight. Some people keep them as a deterent if they keep guinea pigs or other prey-sized fluffy things. I was told this by an old man who breeds guineas.