Are you sure it’s not some addon spamming a log file? What about combat logging? If you have advanced combat logging turned and and some addon is automatically running the combat log command, those files can get absolutely massive.
Check your WoW folder size before opening WoW, while it’s hogging space and after you close WoW. See if the space hogging is coming from there. Otherwise, the only thing I can think off the top of my head is some pagefile shenanigans.