We are lucky that houdini has a pre made node for this called Volume compress if you can use it straight forwards without any addons or with a mask volume as shown below but make sure both ways you tick the (Use 16bit option). 

Why to use it with a Mask group, well in your grid most of the time you will end up with voxels that contains data like velocity (which is so expensive on the hard drive) but those voxels have no density and wont show on the final output so why to have them. This is where the mask volume come in handy, first we create a (volume blur) to expand the initial sim by an amount of voxels (in my case 2 was enough) then we feed that to the mask input of the Volume compress node and in there in the Mask Group field we choose the channel we blurred previously and we set the minimum amount of density which every voxel that has less will be ignored, be careful here it should be set to 0 to be in the safe side but in my case 0.01 was fine it saved me a couple of MB.