Something worth mentioning for fun. The increased braking efficiency of a smaller wheel may be more a matter of less reciprocating mass than it is of wheel diameter (the same reason a smaller wheeled bike may be easier to accelerate than it's large wheeled counterpart)... there is less mass to 'accelerate'. Also take into account that a smaller wheel has less leverage than a larger wheel to overcome the forces/friction of the brake pad clamping on the disc... so is it really a correlation of wheel diameter to brake disc diameter that matters?

Just having a little fun.