Think of the cross-section of one of these rotors, it's a circle, yeah? We're not interested in the interior of the circle, just the perimeter, which increases

Think of the cross-section of one of these rotors, it's a circle, yeah? We're not interested in the interior of the circle, just the perimeter, which increases linearly with the radius. (We can make the interior out of wire spokes like a bicycle wheel.)