[Formendacil]

Another thing to note is that Aragorn was king for over 100 years. That's an entire century during which his kingdom was more or less at peace internally. All the indications were that throughout Elessar's reign, at least, Gondor suffered no civil war, no major famines, plagues, or invasions.

As regards the baby boom effect then, it could easily have been a several generation baby boom. There is certainly room for four, or five generations to be born in the span of Elessar's reign. So if the population doubled (to pick a number), thus having four children for every two adults, then the population of the United Kingdom would have been 16-32 times as much at the onset of Eldarion's reign than at the beginning of his father's.

With that sort of a population boom in Gondor, you could easily rebuild the dwindling population of that land, as well as send plenty of settlers north to the re-established kingdom of Arnor.