A variation of this are dynamic spatially-hashed voxel grids, where the outer grid is spatially hashed and the grid elements are stored (as needed) as dense voxel grids. The advantages of this are that you get voxel grid-like behavior over essentially unbounded size, so long as the data is sparse (otherwise you would need to allocate a significant number of grid elements as voxel grids and the memory savings go away).

These data structures have been used in 3D reconstruction methods like CHISEL where they can often outperform octrees due to better memory access behavior.