Kinetic energy in an 80kg rider on a 7kg road bike doing 40mph (a fairly conservative speed on a road) = 14,000J.
Kinetic energy in an 80kg rider on a 12kg MTB doing 30mph (pretty brisk off-road) = 8,400J.
Or to put it another way, in energy terms bringing the road bike from 40 to 30mph is about the same as bringing the MTB from 30mph to a standstill.
Tiny rotors on road bikes? I think not 🙂