Why do planets have different size?

Answer 1

The theory for our solar system is that the solar wind blew the lighter elements out beyond 3 astronomical units. The inner planets coalesced out of the heavier elements that remained.

Most of the solar systems we have found so far consist of large gas giants orbiting close to their primary star, so this theory does not account for that.

When you look at the rocks in a creek, you notice that some are smaller and some are larger. Lots of them are very small, and a few tend to be very large. There is no reason they should all be the same size.

Answer 2

Because there's nothing that makes them have the same size.

But there are physical mechanisms that determine the approximate sizes. For example a large planet would not be likely to form near the Sun because tidal forces would tend to break it apart and in any case the orbital speeds of elementary fragments is higher near the Sun. On the other hand at large distances the elementary fragments would be so widely spread out that they are less likely to coalesce into one or more planets.

These factors will give small planets close in and far out from the Sun, with larger planets in the intermediate zone.