If you have that from Mejzlik, it can be answer for the difference. those mine are closer (I mean 11" and 12" narrow), usually I not need change throttle too much if I change them. But also two props of the same type have slight differences, so it can happen.
Yes, larger motor is definitively better then any heat sink, it not only absorbs and radiates easier, it usually also comes with lower IR, but makers usually make too large steps between them, so sometimes it is easier to use some cooling instead of 10 or 20% heavier motor. For example AXI 2826 weights only 180g and MVVS 220g and plus it has larger dimater so it is too much for such experiments :- P