I guess this is the right forum, and this may be a silly question - but I've just recently noticed the the rear rotor (at least the part that is showing) seems to be smaller than the front. What up with that????? Enlighten me pls.

 

The majority of braking is done by the front wheels due to the physics of the situation (80%+ or so), thus the back rotors do not need to be as big.

 

Yup, the MINI has 10.9" front rotors that are vented, and 10" rear rotors that are solid. It is perfectly normal for the rear rotor to be smaller, as it doesn't have to stop near as much weight as the front. This is why Big-Brake-Kit's are only really needed for the front; as you decelerate, the vast majoritiy of the vehicles' weight gets transferred to the front axle, putting all the stopping requirements on the front.