in Asimov's Foundation series, there is (was) a galaxy-spanning human civilization, but Earth was lost and mostly considered mythical. One of the arguments against it actually existing was the absurdity of such a double planet system existing. In the wider galaxy only giant planets had such large satellites.

And Titan is 6% bigger than Mercury.

And the 2006 decimation of planet that made Pluto not a planet explicitly added "must not orbit another object" to stop the moon from being a planet.

The earth's moon is an extreme outlier to our knowledge. Thea made our world weird