See this chart and go to the 70F line. That line says that at 135 psia (121 psig) 75 lbs of liquid volume will convert to .4027 lbs (6.4 oz) of vapor at the same volume. So even if a 30 lbs cylinder was capable of holding 75 lbs of liquid, the vapor in that cylinder would only be 6.4 ounces, and that is at 121 psig.
Am I looking at this wrong? I just want to figure it out, and I don’t know where you are getting your density @ pressure numbers from?