The disc brakes have different categories....different caliper sizes..etc. etc.
I am sure the brakes on 125 are there to stop the bulk of a 125 and not of a 250cc...like you can have the examples of 1990-92 Kawasaki GTO's looks how succesful the disc brakes were on that bike.
@Saifi....the problem you said that while braking hard you might end up under your bike...well that depends on the braking technique....you gotta have just the right combination of the rear and the front brake so that you wont fall over and yet stop.