As long as their is some liquid propane left inside the vessel the psi will change with the temperture.

Makes no difference what size the vessel is, 20lb-1000lb same temperture same pressure.

At 80 deg F the pressure is about 80psi.

Also the distance to the tank is involved, the farther the less PSI depending the amount of Propane in the tank. Remember the temperture is a factor, liquid propane will actually freeze in the line.

Tank pressure in propane tanks depend on the temperature. Warmer temperature will increase your psi, while lower temperature will decrease psi. A smaller tank will tend to freeze up faster than a larger tank and your psi will be reduced. In any propane tank, the tank is filled to 80% capacity with liquid propane. A 20 lb tank can actually hold 24lbs of propane and a 100 lb tank can hold 120 lbs. Yet that 20% space is needed for gas expansion which ultimately gives you your psi depending on the temperature. If you overfill a tank and the temperature increases, your tank may rupture. Propane tanks are rated to 300 psi, but rarely go over 125 psi. Your problem is not in your tank size per say, but the temperature, or the setting of the regulator. Check the regulators setting. Make sure it is set correctly. If it is, then you may have an old or faulty regulator. New regulators cost around $25.00. Your regulator is the most important componant reguardless of tank size.