Well, to make my point a bit clear, it's not just about map size.
Map size means more cities to produce more units. At the same time, more units means larger stacks that the AI is unable to use in a proper way because the defending civ also has a larger stack. In the case the attacking AI succeeds, it conquers a city that is only a border city. It would take a lot of the AI to conquer an entire empire. It's much harder to balance the game. The Kalinkan guy for example has to travel a bigger distance to threaten my city, which makes him less dangerous because a) I have more time to set up a defence, he can't blitz me and b) makes him less opportunistic. That's only in theory though.
More units also means more calculation going on. The large desert areas can't be settled but there are still units wandering around, either barbarian or scouting units. It's funny to see how far those scouts can go once you tech to map trading.

Combined with more cities, this means a considerable slower game between turns. I know there are 4 different starts but tech-, map- and unit-wise the starts are entirely different.
Add to that the time to play a turn. Some people like to spend 10 minutes per turn to move all their units and click on new buildings to construct, but it also means a considerably longer game. I like to play a game of civ for about 3 to 6 hours, there has to be stuff going on. When the game takes about 10 to 12 hours (a bit extreme perhaps but you get the point), there may be hours when I think, I'm going to do something other than Civ. Not necessarily bad, but with so many civs on the map I prefer to finish one and then start with another the day after.
That's just my two cents. If the new map is finished I'd like to try it, some more cities wouldn't hurt too much and loading times are even after 500 turns very reasonable. Just make sure that my concerns don't become reality, because the minimalistic approach like RFC has (very small portion of settled land, smooth game with lots of action, invites to replay) is in this mod as well and I'd like to keep it that way.
Don't get me wrong, I like new maps and especially larger maps like most other people do. It's just that in general larger maps tend to provide less dynamic gameplay than smaller maps (and this map is not
that small).
It has to be said that I'm opposing the more = better camp. I don't know if any of you has tried Rise of Mankind: I played one game and it's fun as long as you keep unlocking new stuff but when you're done doing that, the game is utterly boring and can't ever be balanced. DoC seems interesting to me but I wonder if it's the same experience.