I understand the hotpipe, that makes perfect sense.

well, say the intercooler inlet/outlet are 2.5" and the throttle body is 3. So you transition from a 2.5" to 3" diameter pipe which creates lag right. Now if you just ran the 2.5" all the way to the throttle body your not creating any restriction since the throttle body will use/see all 2.5" diameter of the air.

Thanks,
Tyler