As for the other maps that are not in the pool, yes, when these maps are chosen, the population of the server drops.
We have run all maps in rotation, but it is the players on the server who consistently vote for the same maps again and again, hence the current map pool. Not surprisingly, these maps are the ones that have been proven to work on SND (see comp pool).
With the voting, there is no point to add more maps, as they do not get voted for, or if they do, it will be a vote for novelty, a few rounds are played, then the playerbase realises that downpour is terrible on SND, angrily types "shit map" and leaves.
If we were to add more maps, then we would have to consider to removing the voting system, and force the rotation to ensure diversity. However this has been discussed before, and the hit to both the amount of players, and server metrics is not worth it.
As it stands, the map pool will stay the same as has been proven to be the requested pool as per the playerbase.
This does not mean however that myself and the admin team are not open to discussion, and would be delighted to be presented with different viewpoints.