I'm pleased to announce the Purge Temporary Files option (Tools menu) will delete the FMETable directory in FME's TEMP location after a recent update. Please note that using this option while a translation is running could potentially cause the translation to fail, if it removes files that are needed.
You will find the update in our latest FME 2018.1 beta: safe.com/beta
I found the exact same issue. But I was able to get the file up to 729 GB! I am in the process of submitting another case.
Thank you both @erik_jan and @david_r. I have submitted a support case as suggested.
Further to what @erik_jan says, could it be that you've had a lot of crashes or failed runs lately, that prevented FME from cleaning up after itself?
Another thing to try, is to execute Tools -> Purge temporary files in FME Workbench and see if the temporary files disappear.
FeatureJoiner creates a temporary database to do the joining faster.
But that should be cleaned up after a successful run of the workspace.
Please submit this case for further review to safe.com/support.