A good way to tell if an axolotl has the proper weight is to look from the top. In a healthy axolotl, the sides of the belly are as wide as the head, but it is okay if it is slightly smaller. If you are looking to lose weight, feed less meaty foods such as earthworms and feed more pellets/bloodworms. I usually also keep ghost shrimp in with my axies, and they seem to enjoy it.