Gas molecules generally tend to be very spread apart, and not compacted tightly together (like solids). That means you can "bring" the molecules closer together by compressing them into a smaller space.
If you don't understand, think about a train. If 2 strangers got on an empty carriage, they would probably sit pretty far apart (maybe even opposite ends of the carriage). This doesn't mean that 2 people can fit in the carriage only if they sit at opposite ends though - you can bring those 2 people together and make them sit next to each other, and they'd take up much less space.