To check if your rabbit is in the proper weight, rub your hand over his or her spine.
You should be able to feel it easily. If it's hard to find or you can't feel it at all, your rabbit is very fat, and needs to go on a diet!
If the spine is poky, sharp, or seems to be very prominent, your rabbit is underweight. If feeding him more doesn't help, it could be a sickness or possibly worms.
Obese rabbits have many problems and often don't live as long as normal-weight rabbits. They have a higher risk of cancer, heart and liver problems, and often get sick easier then normal-weight rabbits.
When putting your rabbit on a diet, be sure to not change too rapidly. All changes in a rabbit's diet, from introducing new foods, to taking some food out, should be done gradually. I suggest taking out 1 ounce of pellets per week and cut back on treats. If you normally feed some oats daily, change to feeding them twice a week. This applies to all fattening treats in a rabbit's diet. Don't take out hay or veggies, if you are feeding any. Increase the hay intake if taking out the pellets just makes your rabbit hungrier. Most varieties of hay are not fattening. However, alfalfa is, and is never recommended for a rabbit's primary hay source. I don't recommend feeding it if you are already feeding an alfalfa-based pellet.
Rabbits that are underweight are commonly this way because of being diseased or wormy, rather then not being fed enough. To increase the weight of your rabbit(s), give oats or black oil sunflower seeds. Feed half a cup of pellets, if you were previously feeding less then that.
If after a couple weeks and the rabbit is still underweight, or appears bony and malnourished, he or she needs to be dewormed. Please contact me for more details on that.