THE OUTER PLANETS
Jupiter, Saturn, Uranus and Neptune are much larger than the inner planets (Mercury, Venus, Earth and Mars). The question is, why? I believe I have the solution to this problem. See below:
During the initial formation of our solar system, all our planets were close to the size of our inner planets.
The asteroid belt could have resulted from two counter rotating planets that collided to create the asteroid belt. This resulting collision created a lot of fragmented segments to impact on the Sun which produced giant flares that blew large quantities of gas and dust out to the regions of the outer planets. The outer planets then accreted these gases and dusts to grow in size. This process continued for billions of years and that could account for the current sizes of the outer planets.
Today, there is a comet belt between the asteroid belt and Jupiterís orbit. These comets could be formed from asteroids that accumulate some of these gases and dusts as well.
As a result, Jupiterís gravitational field disturbs these inner comets to cause them to impact on the Sun to be the cause of the current solar flaring activity.
This flaring continues to blow more gases and dust to the outer planets continuing their accretions and subsequently, size increases.