1. Propane in the tank/cylinder must vaporize before it can "flow"
2. Normally, if container has not been providing vapor recently, it will be at ambient temperature.
3. As you begin taking vapor from container, the liquid in the container vaporizes and cools. As the liquid coola, pressure within the container goes down.
4. To keep the container pressure up, heat must flow into the cooling liquid. This heat flows primarily through the wetted walls of the container - walls contacting only vapor have a negligible effect. There's a lot more wetted wall when the container is 80% full than when the tank is 20% full.
5. The more wetted area (the fuller the container), the more heat flows in, and the faster vapor is produced. (DO NOT insulate a propane cylinder - this works against you by keeping heat from flowing into the container!)
6. LP appliances require about 1/2 psig (11 inches water column) of pressure. A tank at about -40 degrees F will have this pressure. But if you start removing vapor from this tank - reread 4 and 5.
A larger container provides more wetted area than a smaller tank it can keep the vaporization rate up. A 20# cylinder 80% full will have more wetted area than a 30# cylinder which is only 40% full, and the 20# cylinder will be, at this condition, able to provide greater vapor flow.
Remember - it is vapor coming from the tank, not liquid. (This statement is not true for tanks used with generators or other propane motor equipment).
2011 3500HD Duramax/Allison/90 Gal Aux
2015 HD Limited Low