I will say some words to this debate myself.
In fact it isn't important if one map is some percent bigger than another, neither it is important that there is a need to hunt down enemy units to fight big battles in early game.
The important thing to notice is simply: Balance between factions depends on mapsize and maparchitecture, as well as it does on different units and their strenghts.
To look upon balance without considering the environment in which a battle takes place is a fatal mistake.
Harass is very important. Being in multiple locations and fight over keypoints as well. So if we had a mirror-game on a big 1v1 map, it'll be alright, since bigger maps provide more keypoints to battle for and thus demand more multitasking and anticipatory acting. --> very good for competitive 1v1 to determine the better player.
But the problem I see in bigger 1v1 maps is as already staded, different strenghts and possibilities for different factions.
Take the following example: oddly enough the edain team decided to strenghten riders with forged blades vs. buildings. I don't understand this decision at all, but lets take it as given.
On big maps, harassing with kavallery is not to counter at all with specific factions. In a few seconds upgraded horsemen take down outer farms, which leaves the opponent with far less income. Countering such harass is for many factions impossible, take the elves for example.
So, I hope it became clear what I wanted to say.
In fact I like the Idea of Elendil. We can keep one bigger map in the mappool (Morgai for instance), to support wise faction-choosing depending on map, and to support metagaming. More than this is silly, because of my points stated earlier. As Elendil proposed we could take the gap of rohan out of the mappool and replace it with mountains of angmar.