No, all non-core territory counts towards periphery population. The main benefit of settling historical tiles is to avoid further penalties from settling ahistorical sites.Leoreth, does historical territory get counted as core population, as I kept having troubles in an Incan game, where I literally had all my territory in historical territory up to the point I decided it was time to kick Spain's ass in the 5 turns they had til they got rifling(which I had managed to get before them, through massive useage of gold and spies, followed by simply getting the tech researched first), yet I had over half of my population as non-core pop. Which only got worse when stopped whipping my people to get things built and my cities ballooned to near 14 pop for many.
Inca might be another civ that's disadvantaged by its small core and lack of food in it.
Oh, okay, then it's using what you refer to as size ("population" in the game code, while the "realistic" numbers are referred to as "real population").size is city's ''game'' pop, around 20-30 but population isn't linear with size, when you bring your mouse to city name you will see; it's calculating differently like 33 size city has 33 million pop, but 34 size city has 36 million pop
it isn't important in game mechanics, only for representing demographics