Question:

Why is it that the 'gas giants' (Jupiter, Saturn, Uranus, Neptune) are so much larger than the 'terrestrial planets' (Mercury, Venus, Earth, Mars)?

Answer:

The gas giants formed far enough away from the sun that when the solar system was forming substances like water, methane, and ammonia were in an icy state. Closer to the sun it was too hot, and these materials were in a liquid or gaseous state. This meant that the growing planetesimals could get much larger, holding onto ices as well as rock and metal. The planetesimals that would eventually become Jupiter, Saturn, Uranus, and Neptune grew large enough that they had gravitational fields that could hold onto very light gases like hydrogen and helium. At this point, they began to get enourmous, much larger than the terrestrial planets, which became mostly just rock and metal. that