As In many natural structures, the size of the "cavities" in coral skeleton may correspond to a fractal law (multi fractal porosity, observed for instance in lavas or in some soils). Assuming this is the case, the larger the sample, the large the possibilities of finding larger holes, hence a decrease in density. This is particularly important when using devices with different spatial resolution as in these cases, the observed mean is dependent on the scale of measurements (see Lovejoy work in McGill or Schertzer in Paris). I am not surprised of the observed difference in mean with different voxel size. Similar effects are also measurable at much larger scales with echo sounding devices in which the beam increases in size with depth.