Question

Why are gases highly compressible?

Answer 1

Because their particles are freely movable have a lot of space among them, by applying pressure the spacing between them get filled and they get compressed 

Answer 2

Gases, on a molecular level are very empty, unlike solids were molecules are very close together. This is why very little gas can fill up a lot of area but when gases are cold down, the molecules come together so, they can form a liquid or even a solid and actually have very little volume