From where you got this formula????
Ok, I give you an example, 1 Faber 55 water liter cylinder weight could be arround 47 - 48 kg, it will contain approx 8kg or 12M3 (12 cubic meter) of gas in compressed form.
Where as if you take Inflex 55 water liter cylinder weight could be arround 62 - 64 kg, but it will contain the same quantity of gas.
Hope now its more clear to you.