Real gases

A real gas approaches a perfect gas at 0 pressure. Attractive forces dominate at moderate pressures and repulsive forces at high pressure - since the repulsive forces have a shorter range.

Hence a real gas is expected to be more compressible at moderate pressures and less compressible at high pressures.

Compression factor

The compression factor is the ratio of the molar volume a gas compared to the molar volume of a perfect gas, at the same pressure and temperature:

From the equation you can see that Z of a perfect gas is 1. Real gases with a larger than perfect volume have   Z > 1, and real gases with a lower than perfect volume have Z < 1.

From the argument in the first section we can expect Z to approach 1 and 0 pressure, be below 1 at moderate pressures, and above 1 at high pressure:

We can see this is true for most gases - H₂ being a commonly-shown exception.