I use Advantage for cats less than 9 lbs as well.—-That is the “kitten formula”
My vet has told me to use 1/2 of those tubes for Vivian (and Rucy when she was alive). Rucy weighed 4 lbs, and Vivian weighs 5 lbs. Originally, my vet said to give Jack, who is almost 9 lbs, a half of dose, but that would not fully kill the fleas, so then she said it was okay to use the full tube. That has not been a problem for him. I do separate Jack and Vivian though for 12 hours as they will groom each other and ingest the flea medicine.
If you have a really tiny bunny, then I don’t know if you need to use less than 1/2. You may need to just consult with your vet. I don’t have to go in to see my vet to ask that question. Do you have a rabbit-savvy vet? OHHH…but you’ll have to get your bunnies weight somehow. You can check your weight on a regular scale, and then check your weight again with your bunny and subtract the difference.
I also would watch carefully over the next few days since you put that flea spray on her and then I’m sure she groomed herself, licking it ..not too sure how good that is for her.