My Cooper weighed about 4 lbs or so when they neutered him. The vet wanted him to weigh a certain weight (I can't remember what) but her main concern was that his little bitty testicles needed to be big enough for her to find.
yeah, as I remember it, the vet didn't have a weight "requirement" but she just preferred he be a certain weight before the surgery. He hit that weight way before the surgery anyway... she was expecting him to be 4 lbs. full grown.