Log File Size and dbcc sqlperf (logspace)
The space used will decrease like that for different reasons based on the recovery model of the database named XYZ.
First of all, the transaction log is &quot;circular,&quot; meaning that portions of it (virtual log files or VLFs for short) can be reused once they are no longer needed.  Marking a VLF for reuse is called &quot;clearing&quot; or &quot;truncating&quot; the transaction log.
If XYZ is in the Simple recovery model, then VLFs will be marked as resuable whenever a checkpoint operation occurs in the database (be it manual, or one of the automated checkpoints that occurs periodically).
If it's in the Full recovery model, VLFs can be reused once the information they contains has been backed up in a log backup.
It's likely one of those two things happening periodically that's causing space used to go down.
If VLFs aren't cleared fast enough, the log file will grow per your configuration / expectations (and File Size will change to the increased size).

You should check out the MS Docs page on SQL Server Transaction Log Architecture and Management Guide for detailed information on how the transaction log works.