I suppose one could argue that it's not exactly historically accurate for small nations to be just as powerful as large ones, but it would be much more fun ingame if playing tall was just as viable as playing wide.
Well, conversely, we have many historical examples of civilizations that eventually grew too big and collapsed. Imperialism is how you get an empire, after all, and it is a form of governance that has challenges to sustainability.
Civ is rooted in the now somewhat-dated glorification of Roman imperialism as the pinnacle civilization to which all other civilizations take a backseat. There's an old-school way of perceiving history as little more than a series of wars. Rome was great at steamrolling its way across continents, which has that certain schoolboy appeal for history buffs. But leaning in closer, you find that at home there were long bread lines amidst steep unemployment, and a republic that turned into a tyranny because the armies that should have prevented that were all far away pillaging.
What I think is missing from this most conversations about expansion is the establishment of some kind of metric for governing optimal expansion. When you start thinking of expansion as something to constrain with hard caps or rigid controls like those we have suggested by the OP, you ultimately wind up with an optimal build template like Civ V's "Four-City Tradition". I'd rather that a civ have a more fluid central mechanism. Think of a civ as having a "stability" rating (or call it "authority", "identify", what have you), which is influenced by things like government type, policies, government plaza buildings, wonders, civics unlocked, etc. Your ratio of cities to stability could influence a variety of factors:
Settlers: Production cost bonus or penalty, and cities with a high stability-to-city ration can start cities with increased populations.
Espionage: Other civ's receive bonuses or penalties to missions.
Loyalty: The range that the capital extends loyal could shrink or grow up to 50%.
Districts: Strong stability ratios could allow a city's district capacity to increase by one, while very weak ratios would require a city to reach pop 3 before they could build their first district.
...And so on. In addition, I would tend to think the government plaza should not be a per-civ district, but rather per-continent. This is another way for civ's to differentiate their approaches, electing to take their time to expand smoothly and steadily rather than ISC-style spamming.
In general, it would be nice to see expansion be optimized into waves, rather than something that should be done whenever possible.