I would be concerned about a "light" tortoise or a change in diet habit. A healthy tortoise often feels heavy, like a rock of similar size. Joe Heinens' Russian Tortoise site may have a growth chart/weight so you can compare...? Has your tortoise ever been checked for parasites/worms? It might be wise to take a fecal in to an experienced VET/parasitologist, be sure to call and find out exactly how and what sample they require. Sometimes eliminating these pests will result in a dramatic increase in weight and vigor. A change in weight or diet may be a symptom of a problem, you are wise to figure out why. Good luck, Vic H.