Here are two possible interpretations:
* Largest by absolute size: In this case, Ganymede, a moon of Jupiter, would be the largest relative to its planet. Ganymede is larger than the planet Mercury and has a diameter of 5,268 km, while Jupiter's diameter is 142,984 km. This means Ganymede is about 3.7% the size of Jupiter.
* Largest by percentage of planet's size: If we look at the percentage of the planet's size that the satellite occupies, Charon, Pluto's largest moon, would be the biggest. Charon is about half the size of Pluto and has a diameter of 1,212 km, while Pluto's diameter is 2,376.6 km. This means Charon is about 51% the size of Pluto.
Therefore, depending on how you define "largest relative to size of its planet," the answer could be Ganymede or Charon.
