Any question answered...

So you're left with the job of the brake converting the kinetic energy of the bike+rider into heat via friction, which will be the same whether the rim is 28" or 20". This is assuming you're using the same pads on both rims and the contact area of pad/rim is the same for each.

...cont...

So, if the braking force at the pad/rim interface is the same (same brakes, same pads, same contact area) then the larger rim will provide greater retarding torque as this will be proportional to rim radius.

So larger rim will brake better for the same force at the brake levers (with the various assumptions made above).