Can’t really help I’m afraid, all I know about overweight buns is fatty liver disease…
But I’m sure someone else will know, and this will bump you to the top, so hopefully someone can advise.
I’m interested to see the answer to this!

My bunny Bam got fat after he was neutered. I didn’t realize until the vet told me =( He needed to lose 400 g (14 ounces) and I put him on a diet consisting of very few pellets, unlimited hay and lots of leafy greens + apple-twigs. I withdrew all fruit, carrot, nuts and seeds completely. He lost about 100 g (3,5 oz) per month. I weighed him in a bowl on my kitchen-scales every week and made notes of his progress. This was in the spring of 2013. He’s kept his healthy weight, when he reached his target weight I began giving him a little more pellets and sometimes a treat like f ex apple-peel.

Health-problems from being over-weight is f ex fatty liver, but I’d assume that’d take more than a few months to develop, strain on the joints possibly leading to premature arthrosis, but more immediately, stomach problems like sticky butt from getting too rich food + getting to fat around the “waist” to be able to reach down and eat the cecals right from the anus, which may also cause or contribute to sticky butt.

Definitely talk to the vet about losing weight – it needs to be slowly – activity will help alot too. Slow going is the way to go with losing weight for a rabbit.

Yeah, sticky butt is a real problem. Later in life, the possibility of arthritis is greater, too, with fat rabbits. If she’s already old and you think she has issues moving, I’d check that right away because it will really make it harder for her to get healthy.

If you’re able to provide her with lots of jumping opportunities, that could help. Making her jump to get to her food can help if she doesn’t like to run around a whole lot.

My rabbit, Tank, was 3 pounds too fat when I got him. I mean he was huuuuuge, lol. A case of head tilt caused him to drop too much weight, but besides that he’s pretty much always teetered on the edge of just a little too fat. He just doesn’t like to run around a whole lot. I think the best thing I ever did for his weight was bonding him to the younger rabbits (I don’t know if that’s an option for you or not). The twins like to hop around quite a bit, being younger, especially between levels in their cage, and he’s more likely to follow them.

Obviously, more dark greens and fewer pellets helps. Unlimited hay was really big, too–he loved hay so much, and hay isn’t fattening, so he’d eat that instead of chowing his pellets down in 3 seconds.