Storage is allocated from a Thin Storage LUN when files are created and written to a file system. This storage is not given back to the Thin Storage LUN when a file is deleted or the file size is shrunk. As such, the file system must perform the explicit task of releasing the free storage to the Thin Storage LUN. This is performed by the Storage Foundation Thin Reclamation feature. Thin Reclamation is only supported on VxFS file systems mounted on a VxVM volume.
See Thin Reclamation.