How old is your bun? a bun up to 6 months old can have a bowlful of pellets rather than an egg cupful. once they get to 6 months, it should be gradually reduced to an eggcupful.
With veggies, you need to introduce each veg little by little, after the age of 6 months, unless they have been used to that veg when they were with their mam, as they would have had a nibble with mam. If there has been a period of time between leaving mam, and not having veggies, then start a fresh, and introduce veggies slowly. about an inch piece, for 2 or three days, and see what effect it has on their poops.
Unlimited hay is fab. for my guys I get bales of hay from the local pet/horse shop, and I get a bale of hay for £4.50, which lasts around 4weeks, with 6 buns