A normal recommended and designed 12.5kg and 50kg cylinder can carry as much as 15kg and 60kg GAS respectively if overfilled. All GAS cylinders are designed to be filled not more than 80 – 85%. Obviously the bottle is designed to cope with the saturated vapour pressure over a wide range of temperatures. That is why 12.5kg GAS is recommended filled why that 2.5kg space is left unfilled.
So why can’t the bottle be brim full with the liquid?
If you fill your cylinder completely it has no expansion space, so would rupture the bottle if heated. The required space over the liquid gas allows for the expansion, absorbing gas back into liquid form as the pressure in the space goes up.