collin on Dan Davies - "Back of Mind"

Typically 4x games model population as exponential to a high degree, with diminishing return to scale (going from 1 to 2 population is much cheaper than going from 21 to 22 population.) Infrastructure bonuses tend to be linear ("+1 industry per forest tile worked" or "+1 science per scientist.) And then usually you gradually accumulate multiplicative bonuses making it more impactful to get the flat bonus high (getting "+10% science" regularly throughout the game.) The main way you might see increasing returns to scale is "adjacency bonuses". If a Research Quarter wants to border Research Quarters, the first one doesn't benefit anything, the second one gives two adjacency bonuses (each quarter touching the other), the third gives four (the new quarter touching the two previous, and the two previous touching the new), etc, constrained by the number of possible adjacencies, typically a hex grid. And you'll sometimes get Matthew Effect stuff like "bonus science per technology researched". But aside from that, I'd say the default Civilization case is that most individual subsystems have diminishing returns of scale, but bigger cities accumulate more total systems so you get the snowballing returns by combing flat bonuses from different sources and then getting a multiplier.