I don't think it's hard for a big dog to catch a rabbit,,,in fact quite the opposite ,,,,,, I think your dogs and other dogs that won't run rabbits,,but will deer,,,is simply because they prefer them,,,, I honestly think,,,,the dog thinks it's below them,,,and they get a real liking for the deer,,,,,as you say,,,it might be an old inherited thing from thousands of years ago in there minds,,,wolfs natural prey.....
But I don't agree because rabbits are to hard for them