It isn't often I sit and ponder why something works the way it does, I tend to concern myself with how it works and how to fix it when it doesn't, but this exchange had me intrigued so I've just been doing some sums out of interest.
Unladen weight on the rear axle with the car at kerb weight, is around a tonne (varying slightly depending on whether, petrol or diesel, manual or auto), so, as we normally measure pressure in pounds per square inch, we know the number of pounds. Without actually measuring one, I'd estimate the interior diameter of a rear air spring as around 6 inches so to convert that to square inches we multiply by Pi, giving a cross sectional area of 18.85 square inches. 1 tonne is 2204 pounds, so assuming that would be equally spread from one side to the other, meaning 1102 pounds on 18.85 square inches, or 58.4 psi.
Maximum axle weight when fully loaded is 1840 kg, or 4056 pounds, so that would give 2028 pounds on 18.85 square inches, or 107.6 psi.
In neither case is the height relevant, it won't change no matter how high or low the suspension is, the weight and cross sectional area of the air spring will still be the same. It also explains why the system pressure switch operates at 140 psi to switch off the compressor. Unless the car is grossly overloaded, there will always be sufficient pressure available to deal with the weight. This also explains why my 5.6 bar (81 psi) tyre inflation compressor was only just capable of causing my suspension to lift. Doing the sums backwards means 81 psi would be capable of holding up 1530 pounds, ample if the car was completely empty but not enough to lift it had it been fully loaded. Considering my boot carries an LPG tank in addition to a spare wheel, my toolbox, 2 tonne trolley jack, the assorted spares that live in there and it also had 3 people and one persons luggage, I suspect the rear axle weight would have been pretty close to that so right on the limit of what the pump was capable of.
What I can't get my head around is the effect the piston on the bottom of the air spring has. I know that with conventional springs when talking about spring rate we talk about deflection with weight, X pounds weight on the spring causes it to compress by Y inches, but how is that going to change? With the suspension on High, it isn't inside the rubber bit so is having no effect but when lower it is encroaching into the chamber full of air. The cross sectional area, and hence pressure, won't change though, just the volume of air needed to fill the available space will be less. So how can that influence the spring rate?