BMI is a terrible way to measure obesity. It doesn't take into account lean muscle mass, only height, weight, and "build".
My daughter's appetite will suddenly shoot up about twice a year. She eats like a pig for about a month and will start to get pudgy. Then she looses her appetite completely and shoots up two inches. Her RH took blood tests (unrelated to this) and showed me an elevated enzyme that indicates intense bone growth. She said that this is the normal way kids grow. First out, then up.