Hi.
I wrote that in Dom / Conquest games, cities usually don't exceed size 8, because there is nothing bigger to whip than 4-pop-whips, which need 8 population as a minimum size. Often it's even better when cities are only size 6 or even 4, depending on if they do 3-pop or 2-pop whips. There are still reasons to let them grow sometimes, like if waiting for a key-military-unit, but usually the best tactic is to simply roll over the AIs from the beginning to the end ^^ .
Another reason for keeping cities so small is not only that the whip is more effective, but that having small cities also negates the
from Emancipation, which the last remaining AIs could run to annoy the player.
As already mentioned, peaceful games require different strategies, in Spaceraces i. e. one wants to end with size 15-25 cities. Having those cities in a Conquest game though would mean one doesn't whip hard enough, because 20 pop could be 10 units and 10 units could be another city which again are 20 pop a.s.o.
.
Hth.
[EDIT]
And yes, the capital is an exception, as is the GP-Farm. Best if you also read the next article on city-specialization (signature or strategy guides subsection) .