TLDR; A random individual on earth is more likely to live in a country with large population. Likewise, a random individual in space is more likely to live on

TLDR; A random individual on earth is more likely to live in a country with large population. Likewise, a random individual in space is more likely to live on a planet with large population. Using this type of reasoning, the author shows that the human (earth) is probably part of a bigger group of species (planets) of similar size. Therefore, we can expect most alien individuals to be of similar size to humans and alien planets to be of similar size to earth.