Two nations having a similar population size and a similar average age will also have a similar GDP size if the cost of living is the same. This is because people consume basically the same things in the economy, they eat inside the economy basically the same things. Take states in Nigeria for example say Anambra and Osun with similar population size, these states will also have similar GDP despite the fact that the economies may be specialized in different directions. There are industries in Osun which are virtually non existent in Anambra and vice versa and as it is between Anambra and Osun, it also is between any two entities. Therefore if the Earth is evenly organized with a uniform cost of living and similar tax regimes, it will immediately become clear that population is the only power a nation has.

GDP is Gross Domestic Product.

Two workers can earn same salary but produce different results.
Two servants can feed on same diet but serve differently.