Having looked in the Maps directory, I would say it is because those are the only sizes. I did not look at the sizes, but there were only a very few listed as multiplayer maps.

Yesterday I took a deep breath and clicked the MultiPlayer.. and no one else was in the lobby . There were two choices of maps - Take them or leave them, I mean, Tiny and Small. When I chose small, game said it supported 16 players, so quite a lot of players who wont be able to avoid each other .

First time I chose about 7 AI players and the game was waiting so long to "get data from server" that I hit CtrlAltDel and the task manager said Elemental aint responding.. so killed the elemental process and started again..

Small map again, this time it was DragonMap or such, and game said it supported 11 players. Me, human and Two, AI, game started, and.... LOTS of resources nearby!!. Gold, Fertile, Tech tiles all within 1 or 2 influence range. Also quite a few monsters, overpowered compared to my puny stat sovereign. Maybe cause me not in the North of America, lag was causing noticeable delay in clicks and resulting actions. Looked at the turn counter, and already its at 27!! when did I click 27 times? :-? Ive already lost thrice (game is auto deciding always in MultiPlayer??) My puny sovereign is in danger of becoming a squib  losing so much essence. Afraid to wander too far..  Now the good news, gold and fertile land was nearby, gold coming in, Tech also built, researched warfare without looking at other techs, spent gold to give my sovereign some Defensive garb.. now I can travel a bit..

Travelled a few squares.. another Shard nearby! and another empire's influence is going to cover it within 3 tiles.. train Pioneer, move it close to the shard, Settlement. Moved another few tiles west.. another bunch of resource tiles!!! another pioneer, another settlment spam. Food is not a problem, neither is gilders.. Time to research other techs.. and game is flowing now, still the click count is climbing - to almost 150 already..  and game is still auto deciding fights for me, archers useless, sovereign has learnt spells so when looking at fight details he is now annihilating monsters in 1 turn using a spell.. 

Finally started learning adventure techs, finding things all over the map.. but no heroes yet, no female to be fruitful and to multiply with.. Tower of essence.. I thought I already built one? that means I can build more than one in MultiPlayer?

Is the MultiPlayer game a completely different branch than the regular 1.09 game..

Looking at the "Games" section of Community, the game I was playing is still showing there, with 4 players, means only 3 AI..