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

Think of planets forming from the protoplanetary disk around a star.

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

During this time 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. In particular lighter elements like hydrogen and helium will only be retained in significant proportion in planets over a certain mass. Once planets have such a mass they will grow considerably, collecting gases from the surrounding disk, while their smaller cousins will tend to stay smaller.

(*) One interesting thing about the orbital path of a planet is that there are #5# points called Lagrange points where the gravitational pull of the planet and star match the centripetal force required to keep a smaller object at that point in synchronous orbit. Points L1, L2 and L3 are unstable, but points L4 and L5 - which form an equilateral triangle with the star and the planet are stable.

Objects in synchronous orbit near L4 or L5 are known as trojans.

The first such object detected in the case of the Earth Sun orbit was the asteroid #"2010 TK"_7# discovered in October 2010 orbiting around the L4 Lagrange point.