[Dantilla]

A larger diameter pipe has a slower velocity of exhaust gasses flowing through it.

Think of exhaust pulses as ping-pong balls travelling down a pipe. There is a high pressure area in front of the ball, and a low pressure area behind. We can use the low pressure area to help evacuate exhaust from the cylinder, and with a bit of valve timing overlap, help suck the fresh charge of fuel/air mixture into the cylinder. Ideally, we want a low pressure area behind the exhaust valve when it opens. This can be truely optimized for a short range of RPMs, and a highly-tuned exhaust can have flat spots in the torque curve where we end up with a high-pressure zone where we don't want it.

Now if we put our ping-pong balls into a large-diameter pipe, they all slow down, and we loose the high and low pressure areas that we found so useful.

Exhaust design is really part science, part art. It can be optimized for one small rpm range (3" for racing), or compromised for a wide range of rpms without dead spots. A street car uses a wide range of rpms, and should have the exhaust designed for that use.

To oversimplify, a smaller exhaust will provide the proper volocity for low RPMs (torque), while a large exhaust will provide the proper volocity only at high RPMs (peak horsepower rating).