If you have data deduplication setup on a Windows server volume and you delete a files the space that file took up is not returned as free space until something called garbage collection has run.
This should run once every week. Mine runs at 02:45 on Saturday mornings, this was the default and since it doesn’t conflict with any other high disk or CPU tasks I’ve left it at that. You can see when deduplication tasks are scheduled by running Get-DedupSchedule.
To force a garbage collection run the following PowerShell:
Start-DedupJob -Type GarbageCollection -Priority normal -Volume D:
You can substitute the volume D: for the volume you want to work on. Once you’ve run the command you can run Get-DedupStatus to see how the task is progressing. Deduplication is CPU and disk intensive.