My understanding is that the composition of each body is different. There's the obvious: More ices and liquids comprise Ganymede, possible since Ganymede is so distant from the Sun. And the not-so-obvious: Mercury is anomalously dense due in part to (likely) having a disproportionally large core of iron and nickel.

I find Wikipedia's discussion about Mercury's density in contrast to Earth's fascinating [0]:

> If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3.

[0] https://en.wikipedia.org/wiki/Mercury_(planet)#Internal_stru...

Mercury is basically a big ball of iron.