[Edi]

Gandalf is right about when sites are generated. Independent province population has no effect on which sites are found, unless it is one of the random special provinces that may appear in the game, in which case they have scripted defenders (Solaris the Paladin for one, the two Great Mother and Lord of Fertility in another etc).

Magic sites may add various mages, units or commanders to the list of units that can be recruited from a province when found.

Also, for mapmakers:

If you use the #knownfeature command, any special buildings granted by the site do NOT appear in the province unless scripted.

For example, upon discovery, the Firbolg Fortress builds appropriately enough a firbolg fortress. If you use #knownfeature "Firbolg Fortress", you must also use the fort command, because the site does not appear.

Also, if a commander has begun constructing a fort in a province and you site search and find a site that would give a better fort (e.g. crystal citadel), the fort will not appear because of the fort being constructed.