That is, isn't the largest object in the asteroid belt an accretion of smaller objects? Perhaps the core of this one is a form of ice, and has started to out-gas?
Wasn't there a comet about 5.5 years ago that balloon up to the size of Earth or larger, as it approached the Sun?
Yeah one current line of thinking is that at least some of the asteroids are just dormant comets. They have enough rock and dust on their surfaces to insulate the ice till something disturbs them or they orbit close to the sun.