That's true, they only included Earth's moon, but Jupiter's Ganymede and Saturn's Titan are both larger than Mercury, and Pluto's Charon is large enough to be included (scientist's are still arguing whether or not to reclassify Charon as a dwarf planet, and it's been argued for years that Pluto and Charon form a binary planetary system), so why not include them in the picture? It doesn't make any logical sense to leave them all out.