By separating the ports from one another in a true header, exhaust velocity is maintained or increased to help extract the exhaust from the cylinder, while also preventing the exhaust from one cylinder from diluting the incoming intake charge in another cylinder. This also helps optimize pressure wave timing. Rule of thumb is that you want to use the smallest diameter that will not hinder high-rpm flow. To a modest extent, a smaller diameter helps keep exhaust velocities high and that is where the largest improvements will be seen in terms of torque, which is what you want on the street. Bigger can be better when it comes to headers, but not at the expense of fitment.