But you probably shouldn't even be thinking about this trade-off until you have a few hundred pops directly owned.Civilizations are playable factions, each of which represents a historical nation, empire or cultural group. How many colonies you should own depends on how many pops you have to fill them: there's a trade-off here between having lots of colonies in order to *make* pops for your empire (which is especially effective when you don't yet have many pops yourself), versus the situation large empires can find themselves in, where near-empty colonies contribute to size but have limited value (including a throttled ability to make more pops, whereas they would make a lot more pops if held by a small vassal). But the overall idea is to get as many pops as possible and then arrange them in a size-efficient way, don't deny yourself pops in order to limit size. You can also eventually streamline your way out of using many ordinary districts (in favour of Ecumenopolis/Ringworld districts, or letting subjects produce basic resources), in which case demolishing the excess districts makes sense. So for instance pop traits that reduce empire size are essentially never worth picking at game start, but they can be quite useful to mod onto your pops later. To expand on the 'efficiency' part, empire size management does become increasingly valuable on a macro level as your empire gets larger: early on, the overwhelming factor in tech cost is the *base* cost of the tech (likewise with traditions), which you need raw mass of Researchers etc to overcome, but as you get larger and larger, the +% from size becomes the dominant factor in how much each tech/tradition costs. This is the attitude favoured by the game mechanics overall.

Empire city slots