Permanent magnet motor verses induction motor

How would the size,power to weight ratio of a Pmm/ecm motor compare to an induction motor?

That is comparing apples and pears.
Yes they are a electrical motor.
The stepping motor in your hard drive is a PMM that has iron in its core so it steps.
The motor in a wind turbine is a PMM with out the iron.
So the last two if you compare on same scale will have a other weight and power.

a induction motor has a coil on the rotor that makes its magnetic field based on that it needs more energy.
A PMM can easy be used as a generator or as a brake.

The different technology has different applications.