curious about air intake on ISX

Can someone explain to me why the air intake is choked to half the size of the air cooler outlet? I'm talkin it goes from let's say 3 inch to 1 1/2 then back to 3 right before it makes the turn into the head.

 

ok I can probably see that but what if you have egr disabled? Because I get smoke about the length of my trailer until the turbo boost's up to about 10-15 lbs then no more smoke. I'm thinking it is starving for air at first till the turbo forces enough air to get air-fuel mixture right what's your opinion on that?

 

yeah I've seen the PDI website but i think I can make my own intake tube