we are talking lever arms and torques here. T= F * L or F= T/L.... as you can see from the second eq. as the Lever arm (rotor diameter) increases less Force is needed to "stop" the same amount of Torque produced at the wheel.

however it can get kind of complicated since there is another sort of artificial "lever arm" created from the head area on the bike to the tire contact patch on the ground. this is a totally dynamic lever arm obviously. as you ride it comes in contact with the ground to varying degrees to not at all.

so just think about the first example.

then of course you would have more surface area for heat dissipation. however, the torque scenario is the main reason

HOPE mono4 that made sense