Here's why:

* Gases are compressible: Unlike solids and liquids, the molecules in a gas are far apart and can be squeezed closer together. This means that the volume of a gas can change significantly even if the mass remains the same.

* Ideal gas law: The ideal gas law states that the volume of a gas is directly proportional to its temperature and inversely proportional to its pressure. This means that changes in temperature and pressure will affect the volume of a gas, but its mass will not.

Important Note: While the volume of a gas is not directly affected by its mass, it's important to remember that mass is a factor in determining the density of a gas. A gas with a higher mass will have a higher density.