The size of the moon ensures that it has some considerable gravitational force associated with its mass. This is unlike asteroids that are not large enough to have significant gravitational forces associated with their relatively low mass. Therefore when the moon was molten in its early years of formation, gravity due to its mass, caused materials to be pulled down towards its center. The molten rock, before it cooled into solid rock, got distributed more or less into a sphere because the force of gravity pulls from all sides from the center of mass.
