Its the same in both the intake and exhaust systems.
A b-i-g pipe gives you the maximum horse power by restricting flow the least. But a big pipe means the gas is flowing so slowly at lower revs that when the pressure pulses that occur as the valves open and close travel out the pipes you get flow reversals. That reduces torque at lower revs. Its not back pressure per se that you want, its to maintain a minimum speed of flow. You have to use a pipe size that gives you a compromise between losing torque because the pipe is too big, versus losing power because the pipe is too small. There's no "right" size exhause pipe, there's the size you choose depending on what you want to optimise for, torque or horsepower.
What Renault did on the intake side in the Sport so it didn't have to choose one of either power or torque is have two pipes feeding into the air cleaner, one with the acoustic valve on it that's opened at full throttle at high revs to get maximum power, and closed at low speeds and low throttle openings so you get maximum torque and fuel economy and don't get a jerky throttle action. And most of you silly buggers then remove it because you think you know better than Renault's engineers.
