>>5152779>Because the world in AlttP has more decoration
Half-true> The lack of any other element except a tree or a rock makes the first zelda looks empty with more space.
The difference is actually that the first LoZ is more consistently dense with content to interact with. Alttp is actually the game with more filler and empty space. But you don't notice the emptiness as much because the aesthetics make it feel more like you're moving through a natural environment.
Another big difference is that LoZ is one contiguous world rather than split into a light world and a mirror-image dark world. In LoZ it takes much longer to travel from the top corner to the bottom corner than it does in alttp because the map is twice as big. The images really emphasize this.
Look at how the middle of the alttp map is dominated by 3 locations: Karariko Village in the west, the Castle in the center, and the Palace area in the east. That's it. 3 things, crammed next to each other with only some tree and river obstacles to try and keep you from noticing. In the LoZ map, looking through the middle of the map you have 6 dungeon locations, a graveyard, mountains, a lake, a forest, and the ocean. That's in addition to all the various combat screens and enemies you face.
Looking at it this way you can see all the empty space in alttp. Kakariko Village is boring once you've been through it once. There's nothing of interest in the Lost Woods once you get the master sword and the mushroom. You spend most of the game walking around the Castle (and the Pyramid). The desert is empty. The marsh is only slightly less empty. Dark World Death Mountain is empty apart from the caves (which are admittedly interesting). Dark World lost woods is essentially part of the dungeon.
And so on. I think if alttp had been 1.5x-2x bigger on the same scale, it wouldn't have felt so cramped and small.