The actual aerodynamic properties of the missile ARE affected by its shape and weight, though.
Ranged (Heavy) is the skill that represents using a bow to fire aerodynamic missiles that are designed to fly a long ways and penetrate into their target because they've been fired with such force and still have a great deal of force because they're so aerodynamic.
So what Stewart is saying is that they picked Ranged (Light) to represent firing these missiles that have blunt, heavy tips that are not aerodynamic and aren't intended to be fired with such great force, because all they need do is connect with their target to explode and do damage.
Now that I understand that, it makes sense to me. To be honest, I'm a little surprised they felt that was worth distinguishing two different skills, but the explanation works for me.