Here is what I think I know about this, mostly as derived from first principles (I am a software engineer, not mechanical or even electronics.) As far as I know, it depends mostly on precision.
The closer you can get the electromagnetic field to the counter-magnetic field (permanent magnet for BLDC or DCGMs, rotor winding in AC induction motors) the more efficient the motor can be.
Additionally, the stronger the magnetic field, the more force you can transfer. Force and torque are related; for the same geometry, if you get more force, you also get more torque -- they are not inversely related in motor construction! It is gear boxes that then lets you convert from one to the other.
To get a stronger magnetic field, using better ferrites, using better precision (== closer gaps,) and using higher currents will help. Ferrites improve efficiency, precision improves efficiency, and current improve actual power input. To be able to take high currents, coarser windings and better cooling is used. There is probably some kind of diminishing return if you get too coarse windings for the size of the motor, as the coils won't physically fit at some point -- and more loops of winding generate more magnetic force if I remember my physics right.
Because magnetic forces abate by one-over-distance-squared, even a small improvement in precision can give a significant improvement in performance. For example, improving a gap from 1mm to 0.5mm would quadruple the performance at this level.