It is easy for any animal to slowly gain weight over time, without it being easily noticed, and so any help in assessing your pets body condition is helpful. Rabbits are particularly difficult as they have large abdomens naturally, they sit snuggled up, and they often have thick fur. The differences in size and breeds make it difficult to state a correct weight for each rabbit unless they are purebreeds. Regular weighing, on bathroom or kitchen scales,once they have stopped growing, helps to alert the owner to weight gain or loss, and can provide a useful target weight guide. Assessing body condition is more useful: feel your rabbit over hips, ribs and spine, to detect a reasonable but not excessive amount of fat under the skin. Many older feamle rabbits will naturally have a dewlap under the chin, but if this is too large, or present to more than a small degree, in males, it is due to excessive fat being laid down.
Overweight rabbits may suffer a range of problems. They will neither want to eat, not be able to reach, their caecotrophs, and may become dirty around the tail. This can cause skin soreness, and ultimately end in flystrike and death in many cases. The increased weight can lead to sore feet, which may bleed and become infected. Their joints are under more strain, predisposing to pain from arthritis, and their hearts and blood vessels are worked harder, increasing the risks of heart attacks. They are high risks for anaesthesia, and may not survive surgery that a slimmer rabbit would be fine with. If they stop eating, for any reason, fat rabbits deteriorate faster than slim healthy ones.
Prevention is better than cure. It is easier to keep weight off than to lose weight. The key to maintaining a normal weight is a healthy diet and exercise. Diet is probably the most important, and rabbits are adapted to a low calorie, high fibre diet such as grass. Providing fresh, tasty grass and hay provides all the nutrients a healthy rabbit requires. Providing more than a minimal amount of relatively high calorie pelleted food is the equivalent of that extra chocolate biscuit or 2 every day for us. Most rabbits should get no more than an egg cup full of pellet, twice daily. The exception may be older, underweight rabbits, and/or those with dental problems, and so you should check with your vet before changing your rabbits diet. Increasing your rabbits opportunities for exercise is also helpful: giving them ample space to hop and run in all the time, and encouraging play and foraging behaviours by ensuring that they have a companion rabbit and spreading toys and pelleted food around the space available.