exhaust fallows some weird fluid dynamic laws, because it's pulses not a steady flow, it's true that if it was a steady flow then larger would be better, but gas engines rely on scavenging to operate correctly, that is that the one exhaust pules helps to pull the next pules, just like a musical insterment, you get a guy with little lungs on a tuba and he can't keep the flow rate up to produce a tone, a tone that is produced at the lips by pulsating the air.
If you would like to see an exhaust calculator you can look at
Autolounge.net | Calculators | Exhaust Pipe Sizing you just put in the engine size and speed that you want it to run at, same rules for high speed high power engines as for low speed high mileage engines, after all it's just math.