That guy who hit the 9 footer took TWO hits out of it. He didn't clear it in one hit.

And theoretically, if a bong is past a certain point, then the amount of air you would inhale before the smoke even gets to you would ultimately result in less smoke in your lungs total. Even if someone COULD clear a 9 footer, a 2 footer would most likely be more effective because soon after inhalation, the smoke is already filling their lungs... then they could just inhale until their lungs can't hold any more smoke.

My point is, the larger the bong, the more smoke fills up the bong... but an ideal size bong will fill your lungs with more smoke rather than the bong, thus being more effective.