Stay small because of a lack of material. Disperses the rest into the Kuiper Belt to become Kuiper Belt Objects. Comets and other Trans-Neptunian objects are the leftover icy planetesimals from the formation of the Solar System. The whole planetary assembly process probably took about 100 Million years. Light from the Sun dispersed the remaining gas in the Solar Nebula gas into the interstellar medium.
Planetary motions reflect the history of their formation.
Planets share the same sense of rotation, but have been perturbed from perfect alignment by strong collisions during formation. Rotates in the same direction with its axis aligned with the plane of the Solar System. Planetary compositions reflect the different conditions in which they formed. These are the basic outlines of our current understanding of how our Solar System formed. The basic picture seems sound, but there are many details to be worked out.