Steve,

The really simply way to think about it, is if you look at the compressor map you can see the efficiency areas for given air flow and pressure differentials. If you for sake of arguament could increase the engine capacity by 50% with no other engine changes, the pressure ratio would be lower for the same power at the same rpm due to the increase in swept volume. If you look back at the compressor map for this same amount of compressor flow as before, but now at a lower pressure ratio you may well find you are now in a zone much less efficient.

Ultimately all these means is choose your turbo correctly for your engine, don't just go increasing capacity without considering all other factors.