We typically stick to one lun in one volume. Easier to manage snapshots that way. If you need to put more data in it for dedupe purposes, then increase the volume and lun size in it to what you need. Did you check to see if there are Aggregate Snapshots turned on? Not volume snapshots, but Aggregate Snaps. That will keep the removed space for a period of time. It will also take some time for WAFL to reclaim the space.