Now i dont know anything for certain but this is what i have come up with in my head
The whole idea with the faster moving exhaust (vs. slower moving but larger cross-sectional area) is that the momentum of the gases will create a vacuum behind it as it's leaving between exhaust pulses. This literally "sucks" out the gas from the next cylinder when it's exhaust port opens. That's why a well designed header will give you more power, the exhaust pulses reach the collector just as the other cylinder opens, effectively sucking the exhaust gases out of the other cylinder. This reduces the work the piston has to do on the exhaust stroke, and you get more power
If it's moving slower, true, you should get the same mass flow rate, assuming no compressibility effects. But, although small, they are present, and can give you a nice boost if you use them properly.
On the other hand, you make the exhaust too small, yes, it will flow fast, but you will get alot of energy loss due to friction, and higher pressure will make it harder for the pistons to pump the exhaust out.
Am i on the right track ?