It is my understanding ultimatly the engines power/boost will define the CR.

A low power engine can run a higher CR, such as 8.3:1 for example to mainatin drivability, and a massive power engine, due to the laws of physics (dynamic compresion and flashpoints etc) will require a lower CR.

Exceptions being WRC cars which dont defy physics, their conditions are altered so they stay within the boundrys, ie, boost tailing off at high rpm (Hence they can run 2bar at 9:1CR)

Thats my brief understanding