Why do planets have moons – and why do they have moons which are smaller than the host planet?
Small objects can orbit larger objects if they start out with enough velocity (speed) relative to the large object. They always fall-in because of gravity to the larger object but their original speed keeps them orbiting-falling-orbiting-falling. Our Moon does this. Our moon might have been kicked-up from Earth via an early Solar System collision and kicked with enough energy to orbit Earth. You could ask “why does the moon (smaller) orbit Earth and not the Earth orbit around our Moon?” Actually the Earth does orbit Moon – but it’s a little circle (smaller than the Earth radius) that Earth goes through. The Law of Gravity says that the smaller object zips around the larger object or else falls-in. The larger objects orbit the small object but just a little. Imagine a dance with a giant swinging a little person. The giant would circle a little around his footsteps and the small person a lot. That is the way it is with the Earth and Moon. Also with the Sun and planets.
