This has been studied and formulated and there are calculators out there like this
Autolounge.net | Calculators | Exhaust that tell you exactly what size exhaust pipe you need!
Back pressure is caused by a blockage and not by a properly sized exhaust pipe.
To take it to the extreme if you take your exhaust pipe off your car all together your engines power output will drop, an exhaust pipe is not a duct to the back of the vehicle that is made small to reduce cost, it is a part of the tuning of the engine, that is why the exhaust pipe calculators ask for the speed that you are running your engine at and the displacement, because if you have a huge V8 running at 8,000RPM it is fallowing the same rules as a 1.5 liter engine running at 2,000 rpm.