Dunno why it would be weird considering that you don't see the whole map at once in the PipBoy. All you ever see is one very small square section defined by the limited viewing window, so the overall shape of the map is pretty irrelevant in that regard.
As for leaving the rest for modders, that seems highly unlikely. Half the area they did put content in is essentially empty, which is what modders are focusing on (i.e. putting interiors in all those boarded up buildings, etc.).
I have no problem with them compressing what content they have into a confined space. The less aimless trudging through empty terrain the better in my opinion. It just seems an odd play to include that extra unused space.