Ultima III: Exodus might look like it has a small map, and it some ways it is, but the developers really squeezed everything they could out of this relatively small space. Because of the line-of-sight feature, you can't always see everything that's on the screen. This makes exploring more exciting because you actually have to walk around the mountains rather than just looking at the edge of the screen. The world loops around the edges, but unlike Final Fantasy 2, you can't just walk around forever.
Towns and dungeons can go unnoticed by players for YEARS, without the help of a guide. The same thing happens inside cities, where the line of site blackout can hide gigantic buildings through clever placement of walls and doors. The towns are almost as big as the world map itself, and overall Exodus is a monster of a quest.
Final Fantasy and Dragon Quest certainly have bigger maps, but they've streamlined the RPG to the point where those world maps get eaten up quickly by new players.
Also, FF and DQ are rarely non-linear experiences (although they have their moments). Ultima is basically an RPG sandbox, allowing you to go anywhere, visit towns in any order, shuffle your party around, kill townspeople, steal, and generally have a romping good RPG time.
It's also worth nothing that Exodus was released 3 years before Dragon Warrior and 4 years before Final Fantasy. In those years, it's sad to say, Squaresoft and Enix made the world maps bigger, but the gameplay was undeniably dumbed down and the player was forced on a "quest" that was more like a "hallway with occasional doors." Ultima IV, released only 2 years after Exodus (and still before 1 year before Dragon Warrior and 2 years before Final Fantasy) upped the world map to an insane level, easily topping DQ and FF in terms of realism, complexity, and number of locations. Ultima IV also had a text parsing system which allowed the player to talk to NPCs and get clues on solving the quests. So why didn't other games follow suit with legitimate gameplay advances? Why do no games today use text parsers?