Hay is not the problem — it is the solution
Owners often start a rabbit diet by serving less of everything, which is the one approach a rabbit's body cannot tolerate. A rabbit's gut is built to process fibre almost continuously, and grass hay should stay unlimited even — especially — on a weight-loss plan: it fills the rabbit up, keeps digestion moving and wears the teeth. The calories that built a portly rabbit almost always arrived in the concentrated foods around the hay. So the plan works by subtraction elsewhere: hay stays, water stays, and the measured foods and treats are where every change happens. Expect the rabbit to eat noticeably more hay as pellets shrink; that substitution is the plan working, not a problem.
Cut pellets gradually, and rethink treats
Pellets are calorie-dense and easy to over-serve from a scoop. Reduce the portion in small steps over two to three weeks down to a modest measured amount — many vets suggest around an eggcup-sized portion a day for an average rabbit on a reducing plan — rather than stopping suddenly, which can leave a rabbit hungry and miserable and tempt everyone in the house to compensate with treats. Treats are the second leak: sugary fruit, carrot in quantity, and starchy snacks add up fast in a small animal. Swap them for what the rabbit can have freely in handfuls — leafy greens, herbs, and hay-based variety like a fresh pile of a different grass hay presented as if it were an event. Most rabbits accept a herb leaf with the enthusiasm they once gave a treat.
Movement: make the house the gym
Diet alone is slow work for an indoor rabbit who naps in one pen. Extend daily floor time outside the pen, and make the rabbit move for food: scatter the day's greens and pellet portion across the room or a dig box instead of serving them in a bowl, hide a few pieces inside cardboard tubes, and put the hay rack at the far end of the exercise space from the litter tray so the rabbit commutes. A portly rabbit should build up activity gently — short, frequent bouts of pottering and exploring rather than anything frantic — and a rabbit who is reluctant to move at all may be uncomfortable as well as unfit, which is worth raising at the next vet visit.
Weigh weekly, and respect the danger signs
Weigh the rabbit weekly, same scales, same time of day, and write it down: you are looking for a slow, steady downward trend over weeks, not a dramatic drop. Loss that is too fast is not success — a rabbit whose appetite falls away or whose droppings shrink and dry out is a rabbit whose gut is slowing, and that needs a vet promptly, not a smaller portion. Likewise, a rabbit who stops eating hay entirely at any point in the plan goes to the vet the same day. Bring the weight log and the current portions to each review so your vet can judge the rate of loss and adjust; general guidance gets a rabbit started, but the individual plan belongs to the vet who can handle the animal.
One record for the whole slimming plan
A weight-loss plan runs for months and involves whoever feeds the rabbit, so scattered notes and memory are not enough. Keeping the weekly weights, the pellet portions and the vet's adjustments together in KinsLe's Pet Passport means the trend is visible at a glance, the plan survives a change of feeder, and the rabbit's records packet shows the vet exactly how the diet has changed since the last visit.