I am not sure I understand what you are saying, but yes, I think is what I am arguing. I think.
Head space is the "open air" space in a keg where gas/atmosphere resides. If the gas-in tube, ~8mm diameter, is below the liquid, and this is where the CO2 is pushing, then the surface area of the head space in the keg is effectively the diameter of the dip tube.
The diameter of a corny keg is 203mm. The same pressure of CO2 pressing down on a surface area of 8mm vs. 203mm will push a lesser amount of CO2 per unit time into the beer. Hence a longer carb time.
Unless I am missing something...?