well bigger is pretty much better with all things consistent, which as 1meanz said is pretty much impossible......
with the same friction coefficient pad and same line pressure, you are looking at whatever has the largest and most uniform bite pattern.
in other words bigger is better imo if everything else remains the same which as stated really isn't possible.
i have been wrong before though so i'll wait to be corrected

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btw don't miss understand me meaning bigger rotor or bigger caliper or more pistons. i simply mean whichever has the most uniform bite pattern with, the biggest bite patch.