you are better to have smaller pipe from the turbo to the IC, then big pipe from the IC to the engine. that may seam a little illogical but there is reasons for it.
the small pipe helps keep the volume down and as the gas is already high speed its better to keep it. the air then slows down through the ic (mostly due to volume). keeping a big pipe to the engine keeps the air speed low and less turbulent which makes it flow better. plus your not wasting power accelerating the air again.