Here's why:
* Pluto's moon, Charon, is about half the size of Pluto itself. This makes Charon a very large satellite relative to its parent planet. In fact, some astronomers consider Pluto and Charon to be a double planet system, rather than a planet and a moon.
* The Earth's moon, while large, is much smaller in comparison to Earth. It's about 1/4 the diameter of Earth.
Other planets have moons that are relatively large compared to their size, but none are as large as Charon in proportion to their planet.
