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@Dammit
Loads of factors to take on board when specifying discs- well outside my abilities to describe well, but taking a stab at it:
If you take a look at your average superbike it'll have twin massive rotors at the front, a trail bike typically only one.
Again, a rally car will have tiny brakes when compared to a road car.
The speeds you typically attain on the road, and the repeated heavy stops when compared to "speed modulation" off-road require brakes that can shed speed (as heat, essentially) more effectively on road than off.
Imagine ripping down a trail in Morzine on a downhill rig with 203mm rotors, then doing the same descent on the road- would you really want 140mm rotors to stop you, repeatedly, from 60mph?
I've only overwhelmed the grip of my 32c slick tyres once with my hydraulic 160mm rotors, in a very hard stop down a very steep hill- the two chaps behind me with rim brakes didn't make it and went through the hedge on the outside of the corner.