Propane is delivered and stored as a liquid under pressure.  Propane liquid will expand and become a gas nearly 17 times greater than the amount water expands over the same temperature increase.  As a result, propane tanks are never completely filled with propane liquid.  Tanks are filled to about 80 to 85% of their capacity.  This leaves a space above the liquid, which allows the propane liquid to expand freely due to increases in temperature.