sounds to me that what you're seeing is paintball specific. In general the liquids are low pressure gasses (industrially "LP" means low pressure and not liquid propane). Traditionally, for the high pressure gasses, aluminum cylinders were rated for higher pressures than their steel counterparts ...3000psi (actually higher but 3000 is the rating) and 2250psi respectively. About 10-15 years ago high pressure steel cylinders became real popular in with the compressed air crowd (mainly divers) and they're rated at 5000psi. I'm not exactly sure what the technical difference in the cylinder itself is, but i can tell you there's a BIG difference in the valve and the way it connects to your regulator.
Again, I can't speak for the liquids but as far as the gasses go, tank size (in cubic feet) and pressure are your 2 variables for volume.
so ...if your CO2 tank is steel my guess is that it's rated for 2250psi, if it's aluminum it's rated at 3000 but I have no idea what the max pressure is for CO2.