The Asteroid Belt

One of the theories regarding the Asteroid Belt (located between the orbits of Mars and Jupiter) is that it contains the remnants of a planet that occupied that space. I question that assumption. If true the planet would have been very, very small (only about 1/4 the size of the moon) and not qualify as a planet. Interestingly there is a round planetoid “Ceres” that comprises about 1/3 of the total mass of the asteroid belt. Why is the only body with that shape? Where did it originate? What theory explains the presence of Ceres and the small amount of mass comprising the belt?