If you're a lot too big, you split. It seems to me to be a less annoying way to limit growth than making half your cities useless from corruption or having a hard cap on the number of cities or other mechanisms for achieving this result.
Cause people get ultra pissed off at that 'feature'. Enough that plenty would start a new game or go back to an earlier save. People can understand losing a war, but a Civil War where the computer arbitarily takes half of your empire? A lot of people don't like that at all, in the slightest.
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