I was just playing with another person on DM and we discussed maybe there should be smaller maps for DM. When you have 12 players on a DM (rarer these days than it has been) the maps are large enough to accommodate all 12 players with some breathing room. However when you have less players in a DM (say 2 or 4) then the maps themselves become a bit of a problem due to the overall size of them.
It would be interesting it the match maker could see how many people are on the server waiting to play and from there use a map size that would work for a smaller player base. Of course with the "a player can join at anytime" there is obviously a problem that in the event more and more players join, then the map being played would get too cramped for an enjoyable battle field.
Maybe a different DM option for smaller maps (like 1 - 5 players) and then your traditional maps (for up to 12). Maybe even another DM option for mass players with larger than current maps.
This of course would need new maps etc. being developed and made available in game.