Your comparing using two forces that are not equal. The brake caliper is rated at so many pounds per square inch of holding power ( clamping force), period. Using a bigger ratchet vise a smaller one is leveraging a greater use in power ( moving up for more torque power, like using upgraded calipers ) . Essentially what your really asking here is your stock caliper with it's two piston clamping force to bite down and slow a greater inertia disc and it's not going to be very affective. Better than a stock 11" disc? sure, but not like a Caliper built for clamping down on a 20lbs heavy disc. Plus as I'm sure you also know, the greatest point of force in a spinning object is the most outer side from the center.