Your largest city?

[Lord Emsworth]

Not on grassland. One watered, railed grass gives 4 food. One city center on grass gives 3 food (2 if you're not Agri).

Yes, maximizing food is one reason.

Another is that city centers receive no score for happy laborers. So, if you somehow manage to cover 21 tiles with three cities, that means that you could only get 18 happy laborers working the tiles, whereas one big city has 20 happy laborers working the tiles.

And, yes, that is bean counting.

 

[HagiaSophia]

If you play on the Age of Imperialism Mod, then London grows to size 107