Ok, I'll explain this best I can, apologies if it's hard to make out:  Anyway, the larger the diameter of your wheel, the more leverage available from the rim. True?  So effectively, there are greater forces landing on ledges etc with a larger wheel. True?  With a rim brake, this is compensated for, as the brake also has more leverage. True?  Therefore in theory, as diskbrakes work on rotors of a certain diameter, would they perform better on a smaller rim?  I think it would because there is the exact same amount of power available from a disk brake, on whatever wheel its on, but a larger wheel would have more leverage to work against the brake.  Am I making sense?  Anyone with opinions or greater knowledge please could you learn me!  Thanks beforehand, Alun