The resolution of the voxel buffer is controlled on the Optimisation tab of your cloud layer, using the "millions of voxels" parameter. The main use of this setting is for optimising rendering, but the VDB exporter also uses this number when it generates the voxel buffer.
Try increasing the "millions of voxels" to 100 or higher. For high quality results you might even need values around 1000 or more.
For more information: https://planetside.co.uk/wiki/index.php?title=VDB_Export