Rotor size is your lever arm. Like a 6" ratchet compared to an 8" ratchet, there is a noticeable difference.
It all works together and a bigger piston may have a greater impact than a small increase in rotor diameter, not really certain. Just saying that I think rotor diameter matters more than "some".
I've got a huge spreadsheet that I use to calculate brake torque, but never played with rotor diameter to see how much of an impact. But I do see many big rotor setups with smaller piston area making far more torque than a big piston and smaller rotor setup.
