Why are the Jovian planets so much larger than the terrestrial planets?

Consider how planets around stars form from the protoplanetary disk.

Apart from major collisions, each planet eventually more-or-less (*) clears a path through the debris. Gases and dust in the disk are drawn to one another by gravity and coalesce to start forming planets and smaller bodies. As a planet's mass increases, so does its gravitational pull on the dust and gas molecules through which it passes.

In particular, lighter elements like hydrogen and helium will only be retained in significant proportion in planets over a certain mass; once planets reach such a mass, they will grow considerably, collecting gases from the surrounding disk, while their smaller cousins will tend to stay smaller. During this period of planet formation, the light pressure and solar wind from the star will tend to strip any atmosphere from the forming planets and asteroids, unless they are massive enough to retain it by gravity.

Trojans are objects in synchronous orbit around L4 or L5.

Since the Jovian planets formed farther from the sun, where there was more material available for accretion, they were able to accumulate larger masses over time. The Jovian planets are much larger than the terrestrial planets primarily because of their composition, which is primarily gases, mainly hydrogen and helium, which are abundant in the outer regions of the solar system where they formed. The terrestrial planets, on the other hand, are made up of rocky materials and metals, which are denser and less voluminous than gases.