Things to consider:
a) With water on the rim (e.g. riding in rain), the smaller pad might have a small advantage, since the pressure (P=Fn / A) is higher than with a larger pad.
b) In general, smaller pads are made from a harder/stiffer compound than larger pads. This is due to the different pressure on the contact surface. Since you specify that the pads are from the same compound, either the compound of the small pad is 'right', which would make the compound of the large pad to stiff (as in the pressure would be to low to engage the pad into all the grooves in the rim (Assuming you are using aluminum rims)). Or the compound of the large pad is 'right' and the small pad is to soft (which means it wouldn't last).
c) If you are using aluminum rims, the larger pad might have an advantage, since it "catches" more irregularities/grooves.
d) For prolonged breaking the large pads might have an advantage, since they have a larger area to dissipate the heat.
e) Brake pads work best at a certain temperature (the coefficient of friction between two given materials changes due to many variables, one of them being temperature (that's part of why Formula1 tires are preheated)). Before you reach optimal temperature the coefficient of friction is less and when the pad is to hot the CoF is lower again. (as a side note, melting/burning rubber has a really low CoF)
TH 1.81 (133kg*62)