That would explain a lot. I'm on a small world with a single continent, and presently have something like 68 cities. Part of the reason is that population would grow so fast, and production (e.g., of an aqueduct) was so slow the only way to grow was to create more settlers, so now I've got a map littered with my cities.
Without seeing a screenshot it's hard to say, but if growth is seriously outstripping production in all your 'core' cities, then mine more and irrigate less.
One tactic that was pointed out to me recently (SG linked below) was to place your core cities in such a way that they all have exactly 12 tiles to work -- i.e. can fully employ 12 citizens (the max.Pop per city before building Hospitals -- which you can't do until relatively late in the game, using Sanitation, an optional Industrial-Era tech). This tactic means that at 'full' size, those cities don't need to harvest more than 24 food per turn (FPT), i.e. 22 FPT from the 12 worked tiles, 2 FPT (NB 3 FPT for Agri-Civs) from the city -- so even if you've irrigated most/ all that city's Grass/Plains for fast growth, you'd then convert at least half (more if you have food bonuses, less if e.g. you are already working lots of Hills/Mountains) of that irrigation to mines instead, lowering FPT in exchange for more shields per turn (SPT).
And if you have been automating your Workers, stop doing that! They will 'improve' tiles the same way the AI does -- about 50-50 irrigation/ mines on flatland, with no regard to what you actually
need from that terrain, or in what order. e.g. A city surrounded by Grassland, with all tiles mined, will give 2 FPT and an average 1-2 SPT per tile worked (before Steam/rails) and still grow using the 2-3FPT from the city -- but if 50-50 mined/ irrigated, will only give 0-1 SPT (1-2 SPT with rails) per tile, with a large food excess. At Pop12, that represents 6-12 SPT difference per city, i.e. if you have 20 non-corrupt cities, you could be 'losing' up to 120-240
SPT across your entire empire (it will actually be less than this, because of SPT lost to waste, but you get the idea). Imagine how much more/ faster you could build with that!
In the 'OCN-plus' cities, growth will
always outstrip production/ commerce, because FPT is the only form of 'income' not subject to corruption/ waste, under all govs except Commie (the
only gov-form available in non-modded C3C with 'communal' corruption/ waste -- which means that the production/ commerce from your core will be dragged
down compared to other govs, BTW). As Puppeter says, there's not much you can do to change that (apart from going Commie), so you might as well just farm those towns/cities
EDIT, just to be clear:
'Farming' means irrigating (and later railing) every flatland tile in the farm-zone, and using the massive food excess to support as many Specialists as possible per town/city (Scientists or Taxmen are the most useful once the city hits max.Pop, but you might also want to use CivEngs in the late-game -- after RepParts -- to build any needed improvements).
It's usually not worth building (m)any city-improvements in any farms, unless they will contribute directly to your intended VC, because otherwise they will cost you gold (maintenance) without providing any return -- e.g. Temples and Libs are really only useful if going for a Culture-vic (a Courthouse would be better for lowering flip-risk). A Harbour near a fish would allow a tundra-farm to grow to Pop3 and support 2 Specialists using the FPT harvested by 1 'fisherman' (i.e. the citizen works the fish for 6FPT total, and eats 2FPT, the Specialists eat the other 4 FPT for net 0FPT in that city), instead of a Pop1 town with 1 Specialist eating the 2FPT from the city-tile, and no growth. A 'Duct might let a food-bonus farm reach Pop12 for more Specialists (Taxmen or Scientists) -- but for non-Agri-civs, two Pop6 farms can generally support more Specialists per capita than one Pop12-er.
This has been a slight problem. This particular game has been a learning experience in city management, and I've got zero growth in some places because there isn't enough food. What happens if I literally just abandon a city? Does the population go elsewhere?
No, the city just disappears, leaving ruins, and all the citizens vanish. If you want to rehome them, you'll need to build Workers (i.e. takes 10 turns per Worker in a '1-shield' town, or 1 turn + 36 gold to cash-rush), move them to another city, and join them to that city. (You could build Settlers too, but that takes a lot longer/ costs a lot more.)