There's a whole lot of trickery that can be applied to make you feel like a map is actually bigger than it is. Fallout 4's map is a very good example of that, the downtown Boston area feels absolutely massive because of how dense it is and how many streets and side streets and buildings are at your disposal for exploring, but it's only a very small part of the actual map when you look at the whole thing.
PG didn't do a really good job on that front with Surfers in FH3, it always felt a bit too open. I'm hopeful that Edinburgh will bring a better experience because it's an older city with generally lots of buildings and narrower streets.