Lanefan
Victoria Rules
Ah, that points to one difference between us, then: to me the parts of the setting that haven't seen (and likely won't see) play are not irrelevant, and still do exist in my mind, imaginings, etc. - even when it's not my setting!3) I ran a 7 year, high resolution metaplot/backstory, full-on Setting Tourism game set in FR from late 2e through 2004 with 3.x; full Sim/Immersionist. Even in that game, with high resolution setting and metaplot, there was an unfathomable swathe of setting that had absolutely nothing to do with our play and almost surely never would. But I didn't think of that "unfathomable amount of setting that will never see play as "a grey featureless blob" or a "colorful, featureful blob." It was irrelevant. It didn't matter so it didn't exist in my mind, my imaginings, etc.
When I look at the maps of my own setting or of the one I play in (both are homebrew), I sometimes look at areas that have never seen play and imagine what might be there. Obviously, for my own setting I can take those imaginings and turn them into something more; while with the other DM's setting it's more of a fanciful guessing game as to whether what I imagine being there is what actually will be there if-when we ever visit.
In either case, it's not like those areas don't exist in my mind.