An interesting study has shown that resting energy expenditure in adolescents is lower than in older or younger people. This means that they'll burn fewer calories when resting, leaving more available for growth and weight gain.
I found this fascinating because I was a normal-weight child but became "chubby" at about 11 or 12 and then lost weight with no change in lifestyle when I was about 16. Other family members showed the same pattern.
I'm always interested in why something happens, and in this case the authors hypothesize that because growth requires a lot of energy, the body tackles the problem of rapid growth in adolescence by making adolescents be more energy efficient, so they get more calories from the same amount of food. In a society in which food was scarce, this would be the only way those rapidly growing bodies could get enough calories to build the new tissues they needed.
When growth is complete, the body stops being so efficient with its digestion as the extra calories are no longer needed, and the metabolism increases again.
However, in our world, where food is usually easily available, turning down the metabolism during adolescence may lead to obesity that doesn't reverse when growth is complete.
This pattern of decreased metabolism only during adolescence is obviously not universal. Some people were chubby children and others don't slim down in their late teens when growth is complete.
However, it's an example of the fact that weight gain and loss are not always a result of voluntary food choices. Sometimes Mother Nature is nudging us in one direction or the other.