While Ganymede is larger than the planet Mercury, it is not considered a planet because it orbits a planet (Jupiter) and not a star.
The International Astronomical Union (IAU) defines a planet as:
1. An object that orbits a star.
2. Has enough mass for its own gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. Has cleared the neighborhood around its orbit.
Ganymede only meets the second criteria, it is large enough to be nearly round.
