Interesting, indeed. And it would also be interesting if someone would test a 15 mm hose and a 22 mm hose of the same length to see if the width of the hose makes any difference and, if so, how much.
I agree with the previous posts on pressure loss. To quantify it
There are formulas to estimate the pressure loss but measurement is always better.
The diameter of the hose does make a difference. The pressure loss in the hose is normally proportional to the length of the hose and to the square of the velocity. This may not make sense unless you have worked with the formulas but at any given flow rate if you compare an equivalent length of a 15 mm dia hose to a 22 mm diameter hose the smaller hose will lose about 2.15 times as much pressure as a 22 mm hose. A 10' 22 mm hose would have less loss than a 5' 15 mm hose. there are other losses to consider but these are major ones.
I looked at a manual for a Resp. bipap auto and it says that it automatically compensates for a 6' hose but additional pressure drop will occur when other restrictive elements are added to the patient circuit. It has a graph showing the additional loss from a bacteria filter and when a bacterial filter is also used with a humidifier. It says to use a manometer to verify the patient mask pressure.
I think that this applies more when the machine is in the non auto mode because as was said the auto mode tends to get the pressure right to eliminate apneas.
It seems like especially with older machines you need to read the providers manual to understand how the machine handles the various pressure losses in the circuit. I think I will always try to use an auto machine.