f they have different densities. Imagine a balloon. Now fill it up with air: it expands. Imagine another balloon, and fill it with water until it has the same volume as the first one. The water filled one will be much heavier. This is because the water particles are much closer together; you can fit more of them in the same volume. The gas particles, on the other hand, bounce around with lots of empty space in between them. There are fewer gas particles and the balloon has less mass.