If one searches the forum they’ll see this issue of WD Sync consuming large amounts of the My Cloud free space is a known issue some have experienced. Most likely tied to the fact that WD Sync keeps up to 5 backups of a file it syncs so the user can recover a prior version of that file.
One can use a third party sync program that doesn’t keep backup versions of the files it syncs. Some here use Free File Sync: http://www.freefilesync.org/
Some past threads that touch on this issue:
https://community.wd.com/t/how-can-mycloud-be-full/158401
https://community.wd.com/t/wd-hidden-files-directories/168494
