Well, from what I can tell, it takes a LONG time. They become sexually mature at about 200 grams but I guess they can just keep going. They do max out eventually, though, I have a friend with a Kenyan that's about 20 years old and she's no bigger than 30 inches. I have a 35 inch female, but I bought her that way, and have no idea how old she is. You probably could be feeding them more, but I don't think you're feeding them too little, and it won't stunt their eventual adult size, it will just take them a little longer to reach it.