What backup mode are you using? Is it something with periodic synthetic fulls? If not, you need to schedule a compact operation to free blocks that are perhaps no longer used at all, but still referenced by the VBK. The compact will still use fastclone, but will simply no longer reference any unused blocks so that they can be freed by ReFS. If you're talking about GFS, or other modes with synthetic fulls, blocks will be released by ReFS only once they are no longer referenced by any file.