It means a turbo that size wont be able to move enough air volume to maintain that pressure at higher RPM. Its too small and will be way outside of its efficiency range trying to move that much air. The further you try and push a turbo outside its efficiency, the hotter the air it compresses will be. (Less efficient). And also the faster the turbo will need to spin to do it. So taperring the boost down as the RPMs increase will help with keeping temps down and not stressing the system.

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Doing it for all the wagons out there