@nus if your world is a plain voxel then it's just a bitmap, so they can store pretty well depending on how you encode them, how much resolution they have, and the contents. If you have only a couple of ground materials then you could store them in one bit per voxel maps and then do things like run length encode the bits.

There are also "roaring bitmaps" that are used in data science that are basically a bit of kit that deals with high compression of bitmap data.

If you were storing a material ID per voxel it could get stupid though.