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One of them was a good-sized bit of something, going by this pic of Mimas.
That's No Moon!!!
Most moons have no atmosphere to erode away impact craters, or to burn up even small meteors before they collide. As a result, they tend to be riddled with impact craters unless they are covered in ice. Our moon is about 50 times bigger, so its impact craters don’t seem as large in comparison to itself.