Turbo intake pipe diameter question

[hobieboy]

I'm in the process of finalizing my turbo to intercooler & air filter fitting and have a question...

The inlet on my turbo (where the air filter feeds air to) is ~2.5" diameter.
Most people I know and the K&N filter that flows enough CFM all have a 3" diameter. So it means I'd need a pipe that goes from 3" at the filter end to 2.5" at the turbo end.

Similar situation happens between turbo & intercooler - turbo outlet is ~2.75" and most pipes I can get expects a 3" intercooler inlet.

My question is Why? The big->small pipe from air filter to turbo doesn't seem to do anything in my mind?

Did I miss something?

thanks...