This.
The concept of growing a city to exactly size 20 is completely obsolete, it's way better to build Improvements on what a city currently needs or focusses on, and in Domination / Conquest games, it's often enough if a city has something like 6-8 tiles it can work, because that are the sizes where 3pop and 4pop whips are available. If early in the game, even size 4 can be optimal, because only 2 pop whips are needed.
In Space Races ofc, cities want to become bigger, but even then, it doesn't matter if a city i. e. only has 15 tiles to work but grows to size 22, because then, it can simply hire some Specialists, which have an awesome efficiency once REP is available.
The better concept is to learn, which tiles are worth working and which aren't. In general it can be said, that every Special Ressource is worth working. Grasslands are good to grow on and then whip them away. Sometimes, working more Grasslands is worth it, like when one needs maximum research. Plains are never worth being worked before one gets Biology and / or State Property (Communism) . With Biology and a Farm, they're actually quite ok, but most Domination / Conquest games end earlier than that. Floodplains otoh should almost always be worked, they're almost like Special Resources, because they give Food.
As basic advice one also can say, that the capital should always be cottaged to the max because of Bureacracy, while all other cities can either be farmed (Domination / Conquest) or also be Cottaged (Space Race) .
Sticking to those rules will get you a lot better in the game, as if you would try to grow every city to size 20. Those rules are actually very close to how I really play, after having over 50 Deity-wins under HoF standards and when evaluating every improvement in single.
Are plains cottages in the Bureaucracy capital okay to work? You won't be whipping there [or not as much], so food is less necessary.