The motors supplied with NXT are extremely big. Looking at the internals of the motors, the whole "arm" of the original LEGO NXT motor is just used for the reduction chain. In some constructions this reduction is useful, but in some others it does not at all.
I think a smaller LEGO NXT servo motor without any mechanical reduction (or maybe with a small reduction like the PF M motor) would be extremely useful for many robots.
The image I posted is a badly done photoshop of a reduce NXT motor, but a PF motor with a NXT connector and substituting the DC internal motor by a servo would also do.