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Why is Ceres classified as a dwarf planet rather than a small body?

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asteroid Ida and its satellite, Dactyl — Small body Asteroid Ida and its satellite, Dactyl, photographed by the Galileo spacecraft on August 28, 1993, from a distance of about 10,870 km (6,750 miles). Ida is about 56 km (35 miles) long and shows the irregular shape and impact craters characteristic of many asteroids. The Galileo image revealed that Ida is accompanied by a tiny companion about 1.5 km (1 mile) wide, the first proof that some asteroids have natural satellites.

Ceres is classified as a dwarf planet rather than a small body because it meets specific criteria established by the International Astronomical Union (IAU) in 2006.12

While small bodies—such as asteroids, meteoroids, and comets—are defined as any natural solar system object that is not a planet, dwarf planet, or moon, the dwarf planet category is reserved for bodies that possess enough mass for their own gravity to have pulled them into a substantially rounded shape.23

Ceres is large enough to satisfy this requirement for hydrostatic equilibrium, whereas most small bodies are not. Although Ceres remains the largest object in the main asteroid belt, its classification as a dwarf planet distinguishes it from the myriad smaller, irregularly shaped fragments that populate the solar system.213