I would say detailed, but that kind of depends on what you mean by it.
The level, first and foremost, needs to be well designed. Not just visually, but mechanically, and if you spend 5-10 minutes just running forward to reach somewhere, with nothing to do on the way... Yeah, that's often problematic [Dependent on the game and a bunch else, but lets just assume your normal single player RPG here, like Skyrim, Dragon Age Inquisition, or Witcher 3].
There needs to be something for you to do outside just running forward, and different games address this in different ways. In most though, the easy route is taken, and a bunch of random enemies to fight are put there, and a few generic loot drops. Unless the combat in your game is really amazing, or that loot is really hard to find [Obviously its not but you know...], that tends to just make it feel like grind, with more and more interruptions to the things you want to do.
Others put a bunch of generic quests there to abstract things a level. Rather than there just being random enemies and loot to go to, there is a person telling you to do it. It makes you more inclined to want to do it, rather than it getting in the way of the things you want to do. Not necessarily by a ton, but its better than nothing.
Possibly the best approach to it I've seen is in the old Rareware games. There are combat encounters and obstacles, but they don't interrupt the flow of the game, its literally a one button press to deal with them on your way most of the time, and beyond that your characters had abilities they would unlock throughout the game, that would allow you to access different areas and do different things as you continued through the game. It wouldn't have taken much effort to, for every minute of walking, add something that you would want to use your abilities on to see what was there, making it feel more like discovery than forcing you to go somewhere, and making it entirely your volition to do so. Adding a couple of them that you can't access until later in the game to areas that you need to backtrack through a lot is also often a good idea, as it makes it less mundane to run through the same area all over again. It often isn't much effort to design these little things, however the game needs to support those complex mechanics - which many games don't these days, instead favouring simple hack'n'slash style first person combat, and combat being the only real gameplay element at a player's disposal outside lockpicking. Zelda tends to do this well though, with a variety of tools you can use in a variety of ways, allowing them to hide a lot of things in their world for players to find.
But overall size that just isn't at all used is... pointless, and actually hurts the game. A small level that isn't well designed, or without interesting things to do is better than a large empty level, but it also isn't good and will also often hurt the game. Well designed areas, whether large and open, or smaller and more restrictive, are the ones that you'll remember and enjoy more. Either one done poorly, is not going to be that great a level.
Extra Credit's recently did a 5 parter on level design and the Tower of Durlag. I think that sort of applies to this as well, or at least is a good framework for looking at it. Most designers, I would think, see the "Overworld" as just one area, or an in-between area, and thus it often doesn't have a reward component, or a puzzle component, occasionally a combat component, and usually purely a rather light narrative component. Considering it not as one big area, but as a conglomerate of smaller, open, areas like the aforementioned games tended to, and allowing differences in the content components of each area to keep the player engaged is a better approach than most games take.
While the Arkham games have never been absolutely massive, they have been rather large and I think they address this pretty well. Moving around the overworld itself is engaging to some level. Its not constantly harassing you, its not something you have to really think about, or something you'll usually fail epically at and be unable to continue, it doesn't break the flow of movement either, but you are always doing something, and taking action. Scattered around the world, every rooftop, while open and part of the open world, is its own mini level. Some of them are just resting areas. Some of them contain puzzles to solve. Some of them have a narrative purpose. Some of them have combat encounters. You have a variety of tools Batman can use, and a handful of them are useful just to explore parts of the city, and you feel like you want to do that of your own volition a lot of the time out of plain old curiosity.
So, again, I don't think it matters as much how dense the real meaty content is, or how large your levels are - its what you do with those levels, and how you keep the player engaged, that matters.
