The cells aren't preconstructed, they're assembled as you enter them from the collection of textures.
Let's say you make a room a billion units square and cover it with a single texture. Your file size won't reflect the billion units worth of texture, but only the base texture file you used. So, if you have a 100x100 pixel texture sample, that's the texture size for the whole room. Same for meshes and sounds--no matter how many super mutants you put in there, it's just the one mesh, and just the one set of sounds.
It all depends on the other files. If you make the entire planet and use just existing textures, meshes and sounds, you won't increase the game's footprint on your hard drive by very much at all. Because only one instance of each of them exists in the base files. Your level file is just directions for the game engine to assemble them.
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