In VMware, using VMFS-6 as the datastore filesystem introduces automatic storage reclamation. This process, also called "VMFS unmap" runs in the background (by default at low priority so it doesn't interfere with performance) to clear allocated unused storage space by sending "unmap" commands to the underlying storage so it can reclaim the blocks for other uses.
Sometimes it may become necessary to (temporarily) turn off (or on, if it's turned off already) storage reclamation on a datastore. You can do this through the WebClient but if you want to change this for a bunch of datastores, you can also do this via PowerCLI. While the latter is pretty complicated, William Lam wrote two easy-to-use PS functions to make life more comfortable.