The reason for this - relatively small, I admit - map is that originally during MMO alpha it was never really the idea of developing "YO" as well. Then after the devs figured that the fundraising for the MMO alone didn't bring enough cash in, they decided to develop "YO" first to gain more cash and simply used one map tile (in size) of the MMO map, instead of developing an own map for YO.
So what I'm saying is, it was never really intended that there was gonna be an MMO *AND* "YO" as an full own game with an own map, thus they never made an own large map for YO.
-)
I know that doesn't help the topic as in (how to) make a new map for YO, and in terms of that I agree with you, the YO map is starting to get boring. I was just dropping these lines to explain WHY there is only this one, small map.
-) And tbh, I doubt there will be a larger one to come, because Yo isn't and never will be made for more than 64 players, and for that many plyers, I think the map size itself is actually alright. if you want a larger map with more players, you'll have to wait for the MMO in 2016/2017.
-)