Can't say I've ever seen say, Garrett, change the power rating based on a motor size. They give an approximate of "350-380" for instance as a guideline.
Motor capacity has little to do with how much power a turbocharger can make as it's peak airflow. Airflow which worked out in lb/min from the compressor map taking into account the pressure at that point. A 3ltr motor requires less air to do it if breathing right, or the flip side, run more boost with the smaller capacity motor.
How the power comes on, where torque is made will all be altered by the capacity - but not the flow capabilities of a wheel. It will only flow so much and unless broken, won't flow any less.