You set a large amount of “reserved space” at one point in time.
You can check/change this property of the zvol/dataset from the “Storage” or “Pools” page. In Core it’s under the “Pools” page → edit the zvol/dataset → “Reserve space for this zvol/dataset”. It should be in a similar place under SCALE’s GUI.’
EDIT: For a zvol it might be different. I believe you have a choice to create a “sparse volume” at creation time. With a dataset, the “reserved space” property is not permanent.
EDIT 2: I’m not sure how feasible (or “safe”) it is to shrink a non-sparse zvol. If you outright created a massive zvol, but did not set it as “sparse” during creation time, I’m not sure if you can safely shrink it. It might be possible, so long as it is not currently in use and you do not shrink it below its currently used capacity?
I have no experience with zvols, sorry. I can’t really say how safe such an action is.
EDIT 3: I would stop/disconnect any clients currently using the NAS and create a checkpoint before committing to any risky actions, if you decide to do something drastic to reclaim some space.