People have different frame sizes. You can tell if you have a small, medium, or large frame size by wrapping your hand around your opposite arm just above the wrist. If your fingers meet, you have a medium frame. If they overlap, you have a small frame and probably tend to be naturally thin. Those whose fingers don't meet have a large frame and their healthy weight is going to be a bit more. Sometimes understanding that can help people not to obsess so much about themselves or others beeing different sizes. People naturally vary, and are not meant to all have the same body type.