Thanks for your question. It is important that you speak with your doctor
about this question. Most doctors
will explain the question of weight gain and overall body size with a simple equation of calories in minus calories out equals weight gain. While the "calories in" part of the equation is simple to understand and to control the "calories out" part can be a bit harder. This has to do with more than just the energy that a person uses in being active, but also has to do with the amount of energy that is spent on other conditions, such as growing (height or muscle mass both), the amount of energy spent fighting infection, the amount of energy spent on other chronic medical problems, and a number of other factors. For example, if a person has a cancer or a parasite, both of these will make it somewhat difficult for that person to gain weight. Therefore, failure to gain weight or maintain a healthy weight despite eating an appropriate amount of calories can be a cause for alarm and should prompt a visit to your physician. Sometimes logging the amount of food that you eat can be helpful as well. Please speak with your doctor about this question to get the help you need.