Weight loss can be a sign of ill health, such as dental problems, bowel disorder etc.
Weight increase is usually obesity - check with the vet that you are not over-feeding your rabbit. Overweight rabbits are prone to serious bowel and skin disorders.
I don't think we ever weighed a rabbit in those days or gave any thought to weight control. Maybe fat rabbits were not quite so common - to get fat a body has to be reasonably healthy first, and there weren't many healthy rabbits about. Overweight rabbits are common nowadays as a result of incorrect diet or feeding too many goodies.