I want to ask if you are really serious about the allocation of the race regions in ESO.

The Ebonheart pact has atleast 150% more ground than the other two alliances.
Sure, there is a lot to explore in Daggerfalls, or Aldmeris regions as well but it's not comparable to the Ebonheart pact.

So is it really the developers intention to have such a disproportionate distribution of land?

This game isn't trying to represent the whole landmass of Tamriel. You'll have one (1) entire Cyrodiil, at Oblivion scale, and then a selection of zones from each other province that are presumably roughly equally sized.

Black Marsh isn't open afaik

http://elderscrollsonline.com/en/game-guide/alliances

if you're referring to that ^ map. they've told us that it's just to give us an idea of where the alliances will be. and that the faction territory's are roughly the same size. also, remember that the territory's are zoned and not one huge open area.