I am a huge explorer and have tried getting over the mountains to the west somehow. Well I recently got lvl 20, took the explorer perk, well, there is no locations west of jacobstown (exept nightstalker cave) and the fort is BASICALLY as far east as you can go. You can go far south, and even a good bit of the north is blocked by mountains. Try looking yourself if you don't believe me.
Second, DLC spots.
If you have ever been deep south and even past nipton there is a railroad station like the one where u enter the pitt in F3. U can't go nin but they might do it for DLC. Also, at tyhe mohajve outpost therre is a gate that requeires a key. You might get this key in future DLC and enable the lands passed it, idk. There is also a pool of water in a crater to the very north west with wreckage preventing you from going further. If you examine the map there is still more land passed that barrier, so maybe another spot, or gate, to DLC?
Take this into consideration when you think about the map size and unused territory.