As earlier pointed out here I think it is a mistake to assume that:
1) 1 pop is equal the same number of individuals for every species. The amount of population required to perform various task should varry a lot from species to species.
2) That The number of pops on a planet scale linearly to the number of individuals. I think it make more sense to assign the first pops on a planet (delivered by a colony ship and produced by early growth) a much lower number of individuals than the the 25th. E.g. a 5 pop planet might have 500 million inhabitants, while a 25 pop one have 25 billion.