Good question... but there's more to being a planet than just size. Three requirement were set by the International Astronomical Union (IAU) in 2006.for an object to be called a planet:
The body
1) Is in orbit around the Sun (or star)
2) Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
3) Must be gravitationally dominant enough (massive enough) to prevent anything
of similar size, barring its own satellites, from floating around the
planet. The term that the IAU used was “clearing the neighborhood of
its orbit,” which was one of the most important aspects of the
definition that was debated
Mercury meet all three requirements, so it's classified as a planet.
Pluto meets the first two, but apparently NOT the last one.
Object that meet the first two (but not the last) are now called Dwarf Planets. The object UB313 ( Xena) is a dwarf planet LARGER than Pluto.