It's the same problem. We are capable of creating maps with 60.000x60.000 hex. To compare, Klamath now is approx 400x400 hex. But it's already a pain in the arse just to work with Klamath. I've tested out maps in a size of 2000x2000 some days ago and it was an even bigger pain in the arse, due to performance and overal lack of overview. And really, 2000x2000 is not yet the size that one of the worldmap zones would have.
Additionaly to this comes the graphic problem I've talked about already. Let's take Arcanum... there you have specific mountain walls, that can be used over and over again from _all_ perspectives. In Fallout, there is just front and thanks to modders, the side view. Nothing more. Thanks to this, you would either have to make new walls that don't look repitative or just don't use any mountain walls, which brings me to the next point: The world is flat. Very flat. Not even the real desert is as flat. In smaller maps, like we got 'em now, this problem is more or less not existing, as you never see a really big part of the desert.
Another point is the encounter system. If every zone would be one map and you travel over the worldmap, while you get an encounter, the game doesn't know where you should spawn in the map. So it's either a totally random point or specific points which would be always the same then. Very hard to realize a good effect here.
Also you would most probably be faster with travelling manually from zone to zone than using "fast travel" via worldmap. Always depending on the zone map size and the obstacles in it.
Last but not least, the zones. We have approx 2419 zones. Okay, a lot of them are water zones, but still... Imagine just 1000 zones, this would mean 1000 maps (if you really want to go into detail). All these maps have to be huge, so they will need a lot space, so they will need a lot memory. Also it's very hard to build 1000 maps and _not_ make them look repitative. :>
So to say... the idea is heroic, but very unlikely to ever get done, if one would start such a thing.
I know, I am the destroyer of dreams.