>>28675923
I was hoping you were smart enough not to need a diagram. I was wrong.
So here you go - see pic

Hub B is the same as A, except B has a larger diameter disc and larger pads. The effective radius that the pads operate at for Hub B is greater than that for Hub A

The "average radius" of the pad is the difference bewtween the inner radius of the pad and the outer radius of the pad. If you were working out the torque the brakes would apply for a given clamp force then this simplification of average radius is what you'd use, rather than using calculus