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The theory goes that long, narrow tubes pack air in more efficiently at low flow rates. Meaning low RPM especially. The air in the pipe picks up a lot of momentum being pulled through it, so it keeps moving into the chamber even when there isn't much suction pulling it in.
Conversely, short and wide tubes are better at high flow rates. A higher volume of air can get through the tube in the same amount of time, and that more than overcomes the extra air that gets packed in with that momentum effect above.
So it stands to reason that for any given flow rate (meaning engine size, throttle setting, and engine speed) there is one optimum set of tube width/length that will get the most air into the engine.
When driving for fuel economy, we are almost always running at low RPM. So generally, the long skinny tube approach is better than the short wide tube for our needs.
In practice, there are even more factors that go into intake design. Smoothness will promote flow, and bumps will promote turbulence. Flow is good for getting more air in with less pumping losses, while turbulence is generally good for mixture quality. (And that also gets changed drastically by the throttle position.) And where the air is drawn from, as alluded to above, has a noticeable effect. And on and on.
-soD
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