Yes, I know what you mean. The mayhem is just popular among the masses. The hope is that those that enjoy that type of play will also stick around for more CTF type maps when they are played. Mostly it seems to work, you get a bit of both each day.
What you propose is definitely a good idea. We have discussed it here some before. It will take some changes in the mapvote code.
Recently, I've been thinking that it might be best to still allow the voting of the small maps when there are lots of players but to assign a lower weight to the vote. For example, maybe if there are more than 10 players on the server a vote for a small map would only be worth 3/4 of a vote. The same could be done for extra large maps when there are only a few players on the server.