Grass and hay are great for rabbits because they provide the right nutritional balance and plenty of fiber to keep teeth ground down. A high pellet/greens diet is better than a pellet-only diet, but not as good as a diet of hay/greens with a few pellets. When eating mostly pellets, the teeth don't get sufficiently worn down, which can cause health problems for later on. Either that, or the rabbit consumes such a large quantity of pellets that it becomes obese. Try cutting back a bit on the pellets and offering some really tasty, fresh, green-tinged hay to tempt it.