Um... CFM influences boost pressure, they are not independent. The reason a larger turbo should provide more horsepower is because a larger turbo is capable of pushing the volume of air (CFM) at a higher level of efficiency than a smaller turbo.

The harder you push a turbo too small for a given application the lower the compressor's efficiency will be. Lower the efficiency, the hotter the resulting compressed air will be. The hotter the air, the less power can be produced with that air.