Gases take the shape of their container. When you have a large container, the spaces between molecules (particles) can be further apart than if they were close together. In small containers, the particles are forced to be closer together, or compressed.
Think of it like a pep rally in a gym v.s. a classroom. In the gym, everyone has a bit of wiggle room. With the same number of people in a classroom, everyone would need to be packed in there. This can also explain why a smaller pot over boils from steam before a larger one does, even if the amount of water is the same.
