Turbo's don't make pressure, they just blow air. The pressure that we see on our boost gage is a direct result of this blown air "building up" between the compressor outlet and the intake manifold. While different turbos have different flow capability and efficiencies, two identical engines with the same manifold pressure will have the same net flow rate into each cylinder regardless of the turbo that's being used.
When you hear about a smaller turbo falling on it's face, it typically means that it has reached or exceeded it's optimum efficiency, and you would see a decrease in the manifold pressure when this happens. A large turbo will make more power here simply because it can provide the necessary cfm to maintain the predetermined pressure level as set by the wastegate.
Keep in mind that high cfm almost always means turbo lag. Therefore the ideal setup would be the smallest turbo that would efficiently provide the necessary cfm for your target HP level. Any bigger would just increase lag with a marginal at best increase in peak power.
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