If the temperature and pressure of a system are held constant, the volume of a gas is inversely proportional to the number of moles of gas. This relationship is known as Amontons's law.

In other words, if the number of moles of gas decreases, the volume of the gas will increase. This is because the gas molecules have more space to move around in, so they spread out and take up more volume.

For example, if you have a balloon filled with gas, and you let some of the gas out, the balloon will get bigger. This is because the number of moles of gas in the balloon has decreased, so the gas molecules have more space to move around and the balloon expands.