As per the VMware article referenced in this link Reclaim VMFS deleted blocks for thin provisioned LUNs
The above VMware link provides details for reclaiming unused blocks in VMwares VMFS for thinly provisioned devices.
Unfortunately, this is not automatic and requires use of the unmap command to manually flush this through.
The above article references ESXi v5.5 , but v6.0 and v6.5 also require a similar unmap trigger.