Animals, on the other hand, are designed for efficient movement, and so cannot modify their forms very much. In the higher animals a part of the body that is lost will not grow back. Some of the animals, such as fishes, turtles and some other reptiles, continue to grow throughout their lives. Mammals and birds, however, reach adulthood and then stop growing. Growth is affected by the amount of food available, and an animal or person whose growth is stunted by food shortages during the growing years will never reach his or her full potential stature.
Animals store excess energy as fat.
I'm guessing, but I think it has something to do with the surface area to volume ratio. Each species has an average body size that works well in its environment, and the body's organs, such as the lungs and heart, are designed to work best at that size and weight. Growing larger could lead to a loss of efficiency. This is especially important for warm-blooded animals, whose food needs are high. Also, when the animals become mature, the extra energy that was used in growing can be redirected toward reproduction and the care of their young.