Since the particles of gas all try to separate and move into equilibrium when exposed to space, all of the particles have been moved out to a certain ratio of particle per air, if that makes sense. When moved to a bigger space, the concentration of the particles aren't as condensed. So therefore, the particles will move out and put more space in between each particle. They will all be spaced out, and there will be less total particles of gas per square inch of air.