How is "correct" tire pressure related to tire volume? I have a unicycle with a Surly Endomorph 26"x3.7" tire, and I've discovered that this tire rides best at significantly lower pressure than smaller tires. 

Even though I weigh ~110 kilos, I can ride this 3.7" tire at 12psi without pinch flats.

Previously I rode a 24"x2" Nokian tire that required at least 40psi.

Is there some known relationship where higher tire volume encourages lower tire pressure?
If you ignore the strength in the tire walls (pretty much zero for bike tires) then the pressure (which = force / area ) multiplied by the contact area of the tire on the road must be the force you are putting on the road (ie. the weight of you and the bike).

So for a large tire contact patch you need a lower pressure to support your weight, for a small area narrow tire you need a much higher pressure