Pluto started out being as big as Earth. Over the decades, it shrank to the point that some astronomers quipped Pluto would disappear in the next few years.

Downsizing the "Tenth Planet"

"That's surprising, Brown said, because it means that the object reflects a remarkable 92% of the light that hits it--compared with roughly 60% for Pluto. "I had expected it to be darker and considerably larger," Brown said. Geysers may continually coat the surface with fresh frost, he speculated, although how that occurs on such a frigid body is unknown.

"However, a chart of 2003 UB313's projected size on Brown's Web site indicates that with a reflectivity of 92%, the object would be roughly 1% larger than Pluto's assumed diameter of 2280 kilometers. The team's previous estimates had ranged from 25% larger (on its Web site) to 50% larger (at NASA's announcement in July)."

http://sciencenow.sciencemag.org/cgi/content/full/2006/127/1