Nope, when I adopted Ron he had not been well cared for and was massively overweight he is now a healthy weight. The problem with bunnies and weight loss is that you cannot restrict their food too much because they always need something in their tummy and its not good for them to lose weight too quickly but it definitely can be done.

what is your buns diet? what actually is her weight? do you have an idea of her breed?

Female bunnies need to be spayed or they have something like an 85% chance of developing uterine cancer, is your vet rabbit savvy? How overweight is your bun do you think? it seems a bit odd they wouldn’t operate unless she is really overweight and it can actually be quite difficult to decide exactly what weight a bunny, especially an adopted one that you don’t know 100% what breed the bun is, should weigh so its always an estimate really

Rabbits can lose weight, with a change in diet, plus they need plenty of exercise. Pellets are usually the culprit when it comes to weight gain. You should not drastically change a rabbit’s diet, but you can do so gradually.

Here is one chart showing the maximum daily amount of pellets an adult rabbit should have by weight.

Weight Daily Pellets

2 – 4 lbs      1/8 cup
5 – 7 lbs      1/4 cup
8-10 lbs      1/2 cup
11 – 15 lbs   3/4 cup

The bulk of your rabbit’s diet should be hay, then greens, then pellets. She should always have a supply of hay to eat. You have to watch the treats also, if you do feed treats. Those can pack on weight. If she is an adult and getting more pellets for her weight than she should, you can gradually cut back on her pellets. To see which greens are good for rabbits, you can check out this list:
http://rabbit.org/suggested-vegetables-and-fruits-for-a-rabbit-diet/. If she isn’t used to a lot of greens, go slowly, trying one new kind a week.

Does she get a lot of out of cage time, where she can run around and get exercise? That will help her to lose weight and to tone up.

Unless I’m misunderstanding what you are saying, it sounds like you are unsure if she has already been spayed or not. Did the place where you adopted her from not provide you with that information? If not, maybe you could contact them now. Or, a vet can do an x-ray to determine if a rabbit has been spayed. X-rays are costly, though. Is the vet a good, rabbit savvy vet? The fact that he or she said it’s difficult for rabbits to lose weight, makes me wonder how well versed he really is in rabbit care.