Because the whole lot is resistive, so the more small diameter sections you have the worse it gets, when you think about the mimimum size its going down though, its the ports in the head, which are only about an inch across and its only ever flowing down one of them at once too as its not like all 4 cylinder have their valves open at once.

All these hard and fast rules you keep quoting about x-mm = y-bhp etc are only rough guidelines anyway, stop taking them so seriously