The outer planets are much larger than the inner planets and are almost entirely composed of different gases.
Many of the inner planets are rocky and hot, due to their close proximity to the sun. Meanwhile, the outer planets are composed of different gasses. These gases are much farther apart in general than the solid planets, just due to the basics of solids and gases. The outer planets usually contain a small core which possesses enough gravitational force to hold the gases around the core.
Example: Mercury is composed of 70% iron; whereas Jupiter is composed of hydrogen, helium, methane, ammonia, and water.
In the picture above you can clearly see the size differences between the inner and outer planets.