Why are some planets in our solar system so small and others so big? It’s all due to the sun. The planets closest to the sun became very hot. They also had relatively little gravitational force, so the lightest matter could easily drift away into space. That is why Earth is now small and rocky. The large planets, which are further away from the sun, are made up mainly of gas but have small rocky moons. 
The moon bears the scars of a huge number of meteorite impacts. Because there is no air and no water on the moon, the craters have remained sharply defined. On Earth, most impact craters have weathered away. Photograph: NASA
For billions of years, the planets and moons throughout the solar system have been bombarded by bigger and smaller lumps of rock and metal. You can see that from the impact craters on them. And the bombardment is still going on.