I was just reading some opinions about the map sizes for Fallout 3 and Fallout New Vegas and even though they were both comparable, Fallout 3 had a map that some felt (maybe rightfully so) was more more conducive to open exploration. Locations were spread out well, while in New Vegas the majority of stuff was centered around the highways and New Vegas itself. On top of that, you could get seriously punished if you wandered too far out of a particular area if you came across deathclaws (much harder in New Vegas, which I actually preferred), cazadors, etc. There were also more mountainous regions that limited how one could move around.
The underlining point is in how you can have two similar map sizes but one could be designed much better than another. At this point I think the Warhorse team sounds like they have a good plan for the map they’re going to use based on the game they’re trying to make.