The Earth is a sphere, which means there’s no easy way to project it onto a flat surface. One of the methods used to project the Earth’s surface on the map results in certain places (such as Greenland) to be stretched to huge sizes, sometimes appearing as big as Africa (look up “Mercator map”). The joke here is that while we expect him to make a comment about the map’s projection, he instead comments on how Greenland on the physical map is only a few inches, as opposed to its actual size.

So yeah, subverted expectations, peak Dad joke