Here's why:

* Composition: The gas giants (Jupiter, Saturn, Uranus, Neptune) are much more massive than the terrestrial planets (Mercury, Venus, Earth, Mars). They are primarily composed of lighter elements like hydrogen and helium.

* Pressure: The immense gravity of gas giants compresses the hydrogen and helium at their cores, but the pressure is not enough to force these elements into denser states like liquid or solid.

* Density: As a result, the gas giants have much lower overall densities than the terrestrial planets, even though they have a larger mass.

However, there are some nuances:

* Internal Structure: While gas giants are mostly gas, their cores are thought to be made of rock and ice, which are denser.

* Variations: The density of planets can vary depending on their specific composition and internal structure.

Example: Saturn is the least dense planet in our solar system, even though it is the second most massive. This is because its composition is mostly hydrogen and helium, and its internal structure is less compressed than Jupiter's.

In conclusion, while there's a general trend of more massive planets being less dense, it's not a strict rule and there are exceptions.