If the volume of the gas is decreased the molecules will hit the walls more often and so the pressure of the gas will increase. In fact if we reduce the volume to half the original volume then the molecules will hit the walls twice as often and the pressure of the gas will be doubled.
Reducing the volume to one quarter of the original will increase the pressure to four times the original.

The pressure of a gas increases in inverse proportion to its volume as long as its temperature stays constant. This was discovered by the Physicist Robert Boyle and is known as Boyle's Law.

Original Pressure (P