queenpea
King
Populous covers both IMO.
I think the issue is that there shouldn't be bonuses that help both increase a civilization's population density and the territory covered by that civilization. The two should be somehow opposed, being simultaneously good at both cuts away a lot of the game's dilemmas. It would make more sense, IMO, to have one adjective that applies to civs that are mostly cities with lots of specialists, and another to civs that are mostly towns and/or rural districts. But having civs that are just "good at population" feels too broad, because pop is the most fundamental and powerful resource in any Civ game.
If we are accepting that “Expansionist” civilizations are focused on city growth, then “Populous” is a much better term.
A quick Google search confirms that synonyms of “expansionist” include “colonial,” “imperialist,” and “subjugative.”