This image shows the relative sizes of the major moons in our solar system with the size of the Earth and it's moon. Notice that Europa and Ganymede, two of the moons of jupiter are as big as small plantes like mercury.
It makes sence that the largest planet in our solar system, Jupiter, has the largest moons. Althoug Jupiter and Saturn both have over 60 moons, most of them are too small to be seen on this chart. When you compare the monns in our solar system, most of them are smaller than earths moon.