Map size has literally nothing to do with map quality. Case in point: Spencer Mansion, Resident Evil 1's famous mansion. The entirety of the game takes place inside or near the mansion, having you explore every nook and cranny, allowing the player to become familiar with their surrounding on just a level to memorize floor plans and secret passages. On the other hand, you have games like Just Cause 2, which boasts an incredibly expansive world, but little to no detail to show for it.
Taking the piss out of Fallout 4 for having a small map is unfair for the developers. Having a smaller map necessarily means it's easier to put attention into detail rather than expansiveness, and Skyrim has pretty much shown that a small map can offer quite a bit of exploration.