The low barrel weights are possible because 2" is low pressure cartridge.
Whatever the pressure of the shells, at least initially, 2" Brit guns had to pass the same proof pressure as other standard Brit 12's. Eventually, a separate and lower proof category was added for 2" guns. I seem to recall that might have been as recently as the 50's, but I'm not sure. One reason the 2" guns from Spain are heavier than most Brit 2" guns is that those Spanish guns were CIP standard proof, not at a reduced pressure standard for 2".
