One of the benefits of using RDMs is that SCSI UNMAP is a much less complicated process than using VMDKs. Depending on the version of ESXi you are using there are different How-to Reclaim Space after deleting files on host systems for UNMAP to be successful with VMDKs. With RDM the only requirements are that the Guest OS support SCSI UNMAP and that the ability is enabled.
Windows
If utilizing Windows please refer to Windows Space ReclamationKB for UNMAP requirements and how to verifying this feature is enabled.
Linux
If utilizing Linux please can refer to the SCSI UNMAP KB for UNMAP requirements and how to verifying this feature is enabled.