Once the piston is on it’s up stroke, this air and fuel mix is compressed and something called adiabatic heating occurs. This results in the temperature of the air/fuel mix entering the combustion chamber to be significantly higher. The problem is that the more boost pressure you run, the more the charge is heated by the turbo. Turbo A (smaller turbo) running 1.0bar will spool faster than Turbo B (slightly larger) running the same boost pressure, but the amount of air moved by Turbo B (slightly larger) will be greater at the given boost pressure as it’s moving more air? Thus creating more power.
People run smaller turbo’s because they want them to spool early and to deliver a wider range of power. It’s all about correct turbo choice a lot of the time. If the boost level is low, high CR is fine and will feel nice to drive, but if you go to 10:1 on pump fuel there is only so much that piston design and even the most modern EFI can do for you. Now with 109 RON Sunoco GT PLus fuel, ignition timing at 1.8 bar jumped up around 7 degrees, and we were able to run easily a 2 bar, we would have run it at 2.2 but came up against an ignition misfire. I've mapped a number of high CR 4G63 and they make peak power at about 1.8 bar, but the jump in power from 1.4 bar to 1.8 bar was very small because the advance had to be reduced to very low levels (2 degrees at peak torque, 3.5 degrees ar peak power). High CR really only becomes a problem as you try for high boost levels. Using the R35 as an example, it makes (for arguments sake) 600hp on pump fuel with the right turbos, but the size of the engine means it doesn't need lots of boost to do it. But choosing the right CR is very much to do with the level of boost you want to run.