* First, an unhealthy (and unnatural) diet leads to chronically softer stools, as the gut is not good at handling the refined foods being fed. This results in smeared floors and is one of the causes of the affliction kindly known as "poopy butt".
*Female rabbits will also grow an enormous dewlap, which may become chronically wet from drinking and develop skin infections.
*Some rabbits suffer from arthritis with age, and being overweight only makes the problem come on earlier and with greater severity.
How do you know if your bunny is fat?
Many house rabbits are, and they are not at all embarrassed by it! Rabbits store fat inside their abdomen, which makes just eyeballing them for love handles ineffective. Run your hands along your rabbit's side, feeling the rib cage. When you reach the end of the ribs, your should be able to feel a slight inward slope, the waist. Fat bunnies will balloon out instead. Remember that the ribs are palpable in even quite obese rabbits, so don't let your bunny fool you into more snacks!
Heath problems associated with obesity are increasingly being recognized in dogs and cats, but public awareness of this issue in house rabbits has not been widely recognized.
Despite the progress made in educating owners that rabbits should eat a diet of hay, leafy greens and limited amounts of high fiber pellets, most of the rabbit ‘treats’ sold in the pet stores contain unhealthy amounts of sugars and starches; even the pellets often contain bits of junk food which many rabbits pick out and eat, leaving the healthier pellets behind. Many individuals equate food with love, constantly offering their rabbits high-fat,/high-sugar snacks.

Too, it can be more difficult for rabbit owners (as compared to cat or dog owners) to recognize signs of overweight in their pet. It can be visually difficult to determine if your rabbit is obese, and different breeds of rabbits have slight differences in body shape. Many owners perceive their rabbit’s weight as being just fine, when in fact the rabbit is obese.
Early signs of exercise intolerance and difficulties with mobility associated with obesity are not as easily identified in rabbits as they are in dogs and cats. On the other hand, female rabbits may have a very good BCS and yet some will naturally have a very large dewlap which can make them appear overweight. The best bet in determining if your bunny is of an appropriate weight is to take your rabbit for at least an annual checkup to a bunny-savvy veterinarian.
Get your bunny in for a checkup.
Follow your veterinarians recommendations as to an appropriate weight for your bunny and how to achieve this.
Avoid all the unhealthy junk treats sold by the pet stores, which not only make bunny fat but promote unhealthy bacteria in the gut, increasing the chances of potentially-fatal GI stasis. If you want to offer your rabbit treats now and then (not entirely a bad idea, as refusal or acceptance of treats is one way to gauge whether bunny has an upset stomach or not) offer them healthy treats.
A half a grape or a thin slice of banana is a fine treat. Purchase natural (no added sugar or preservatives) dried fruits or dry your own (but offer these in minuscule amounts, as drying concentrates the natural fruit sugars).
Oxbow dried fruit treats for rabbits are sold locally at PetCare Solutions, or you can buy dried fruits at Liberty Market or dry your own. In the summer, grow your own herbs, plantain and thistle – bunnies regard these high-fiber 'weeds' as fine treats, and they can also be dried for winter use.

Encourage exercise.
Change their toys around, switch out hiding boxes for cardboard tunnels – anything to add interest and get your bunny exploring his surroundings.
Play games with your bunny.
Try putting his pellets in a Go Cat Go toy so he gets exercise while he gets his pellets!
Never withhold food from your rabbit; they have to eat constantly to keep that gut moving and avoid GI stasis. Do feed your rabbit unlimited hay and water 24/7, limit the treats, and get them moving!
PET-SIZE-O-METER:
http://www.pfma.org.uk/_assets/images/general/file/PFMA%20Rabbit%20PSOM%20Final%20Web%20Version%20070809%281%29.pdf
For further reading:
http://www.rabbitnetwork.org/articles/obese.shtml
http://www.examiner.com/article/is-your-bunny-obese?CID=examiner_alerts_article