I easily spent more than 150 hours in Fallout 3 and never once thought that the map was small and I think if I was to play it now I would still find it pretty big. One thing is for sure, Fallout 4 is much bigger than 3,so that's a big plus already. Thing is, we've gotten used to huge game worlds like in GTA V and Witcher 3 for example. But if I'm honest, as cool as the huge part of the countryside is with the mountains and everything in GTA V, I barely spend time there. Witcher 3 is a different story.
However, if Bethesda has indeed improved the density, added a lot of places to explore, I am OK with this. There definitely were places in Skyrim that were huge open fields and barely anything took place there. With all that said... Never ever with a BGS game did I had the feeling the map was small, it probably will be OK with this one too. It is after all their most ambitious game yet.