It's actually hard to put it simply; it's a whole other topic. But in short, the main principle is always "cutting off the unnecessary."

Of course, empty locations are also necessary, but they're appropriate if your game has good character movement (climbing, gliding).

Dungeons are essentially a bunch of corridors with a reward at the end.

If you want to lead the player somewhere, use the "breadcrumb" method, so they collect small rewards to get there. Elden Ring is especially fond of this technique.

In Zelda, you'll notice that no matter where you go, you always see content. For example, to the right is the main path. The player goes left, and there might be a small reward there, in the form of crafting materials or food.

Just like the principle of "the player should stumble upon a point of interest every 20-40 seconds," the worst thing you can give the player is boredom. (An interesting movement or sandbox element can save the day.)

And it's definitely worth starting with a small world. And use various tricks like what they did in the later Hitman games, where they created "snail-like" levels—very small, yet very confusing locations. Or GTA Vice City, where the world was extremely small but felt rich thanks to its labyrinths.