When using EEVEE Volumetrics the minimum Tile Size is limited to 2 pixels. Why it is not possible to use 1 pixel or even 0.5 pixels?
Using higher Global Sampling does not help to refine the VDBs, since the Volumetrics do not seem to react on changes there. (You can check this by setting Vol. Tile Size to 16 px and then render with different Global Sampling - in all cases the Volume is blocky)