A useful script for finding stuff like this:
It will output a list of folders in the current directory and the size of their contents. Run it on / and wait and you should see where the huge size is being used. If this is a ZCS server like it sounds like, my guess would be /opt/zimbra/store is your likely culprit.
du -sm $(find $1 -maxdepth 1 -xdev -type d) 2>/dev/null | sort -g
user@server:/usr> du -sm $(find $1 -maxdepth 1 -xdev -type d) 2>/dev/null | sort -g