You're asking about reclaiming the space, previously occupied by a large VMDK file, from the datastore itself. For this, you will need to issue the UNMAP command to the datastore if it is VMFS-5. Starting with VMFS-6, the UNMAP command is issued automatically. For that, you will need to use the command esxcli storage vmfs unmap to issue the UNMAP to the datastore.