I wouldl of assumed, that the compressibility of something would be determined by its initial density, water is more dense than air, so is harder to compress,
steal is more dense still, and even harder to compress, but any diver or submariner will tell you that water is highly compressible. but being fairly dense to start with, its harder to make it 'more' compressed.
and steal is allready very dense.
If when you compress something, (air, water or whatever), what are you doing? if you are forcing the atoms closer together, then i wonder what is in between the atoms or molecules.
if you can compress air, and put more gas molucles closer together, what is between the molecules before you compress it, (normal air pressue).
are the gaps between the moleculoes a vacuum ?