I used a cat harness for my 3.5kg bunny, but I dont use it anymore because of the following...
Be aware though that you should not walk a bunny like a dog, if you walk them it must be within the house or a secure garden. If you meet a dog or a prey-driven cat it would be dangerous for both you and the rabbit, dogs have been known to push humans to the floor or even attack them in their excitement to get to a rabbit. Also if the rabbit gets scared by another animal, a noise, or seemingly for no reason, they may bolt, and when the lead pulls tight when they bolt they have been known to break their own backs or necks
this can also happen if they struggle in the harness, their bones are so lightweight to make them fast to avoid predators that they also break so easily.
Rabbits are also prey animals and so are naturally very nervous of new surroundings, it is very stressful for them, even if they dont show it (they are designed not to show weakness).
Also, quite frankly, rabbits are rubbish on the lead! :lol: they dont follow you around like a dog, they just dont understand the concept! Again it is because they are prey.
If you want to give your rabbit some exercise outside, you could get a big run for her, so she can run about as she likes in it