While I see and understand this from every angle (business, maturity, and stunting ability), I will take it from another angle and that's actual physical wear and tear. Every additional pound these kids are lifting, throwing, and catching is in direct opposition with gravity and force, there is no such thing as a "normal" size flying human. When it comes to bones, joints, cartilage, joint cavities and their injuries, it always correlates with force and impact. Would you rather lift and catch a 75 lb flying cannon ball or a 115 lb cannon ball repeatedly? At some point, my body is going to stop caring how mature the cannon ball is.