Here's why:

* Inner planets are small and rocky: The inner planets (Mercury, Venus, Earth, Mars) are much smaller than the outer planets. They are primarily composed of rock and heavy metals like iron and nickel.

* Outer planets are gas giants: The outer planets (Jupiter, Saturn, Uranus, Neptune) are much larger than the inner planets. They are primarily composed of gas, primarily hydrogen and helium, with smaller amounts of other elements like methane, ammonia, and water.

So, the inner planets are small and rocky, while the outer planets are large and gaseous.