I just upgraded an old mountain bike with XT hydraulic disc brakes. The rotor on the back is 160mm. The rotor on the front is 180 mm. There is a rather different feel to the back brake vs. the front brake. I just assumed it was because one of these wasn't bedded in as well as the other one. My kid suggested it was instead a property of the different disc diameters.

Which is a more likely explanation, and if it is due to the difference in disc diameter, which brake would you expect to appear more powerful?