Strictly speaking if you were to fit the exact same disc brake system on 700c and 20”, the brake “power” will be greater on a 20” because of the smaller mechanical advantage (radial lever).
Between 50-662 and 50-406, the contact path is greater for 700c (even if considering 40psi of 700 and 26psi on 20).

So, the mechanical load generated by a 700c on a said 160mm disc is going to be greater than what a 20” is going to produce. With the same braking system, if you apply the same load on the lever, the 20” will skid.


Rough calculations would said that for equal caliper, lever and pads, you should get similar performance between a 20” fitted with 140m disc and a 29” fitted with 203mm disc