That is true of a lot of lizards, but not really for chameleons. Iguanas, monitors and some others store fat in their tails, but chameleons store fad in the pads over the head and in fat pads in the abdomen primarily. The best way to evaluate obesity is look at their head pads - they should be slightly convex but not bulging. If they're sunken or flat you may need to feed a little more. If you can see the arm and leg bones then your cham is very thin. The tail may appear thinner in a very thin animal because of muscle loss that comes with malnutrition/anorexia/starvation. But in obese chams you don't really see much change in the tail. With obesity your cham may look chubby overall (fat neck, fat arms, etc).