Speaker efficiency and frequency response, and EQ, of course having lots to do with perceived volume. I should have prefaced my reply with "all other things being equal"Errrr....no.....12" speakers have 44% more area than 10" speakers.
And 40 watts is 40 watts. Whether it's pushing 12s or 10s, it's still the same amount of energy, and the net effective potential volume.
More speakers spread the sound out, or distribute the energy more evenly, which is why a 100 watt full stack with two 4x12" cabinets tends to sound louder in more places than a 100 watt single 12" combo, which will have a much smaller (1/8th) starting point for the sound. Being directly in front of the single 12" will feel like it's much louder than in front of a full stack because it is 8 times the energy per speaker. But if you are listening off-axis to the single 12, it will likely get lost pretty quickly.
Mic-ing the amps and running them through the mains is the real answer for a balanced mix.