From what I’m reading 8n the forums, it appears to be a combination of three problems: bad algorithms, bad defaults and a poor user interface.
One thing seems to be versioning (Cloud Station, as @rdg points out). The default is 32, I had set it to 5 (who needs 32 versions of a file). Versioning is a good thing. But it appears the Synology software can’t tell if a file arriving is actually changed, so every time I did a backup from my desktop it saved a new version. Wrong because both music files and photo RAWs are immutable, write-once-read-many, there should be no versions. And for those things that do change (Roon metadata, Lightroom metadata), the apps (Roon and Lightroom) do incremental backups.
And on the user interface, there is no way to find and get rid of them. I changed settings, disabled versions, didn’t delete the extra versions. Intentionally, it appears, because it didn’t warn me about loss of data.
The recommendation on the forum is to uninstall Cloud Station. What is that? Maybe I wanted some aspects of Cloud Station? It would lose all versions, maybe some financial data had important versions? I did anyway, this time it did warn me, and I have reclaimed 4 TB. So now I’m only 3X over, not 4X.
Some forums have said deleted files are recycled and stay there, maybe but I can’t find that in the user interface.
Some forum members (and @Wizardofoz) recommend going at the Unix level. But I refuse. I bought an appliance, Unix is at an uninteresting level of abstraction.
I’m sure it is possible to use the Synology effectively, if you learn it. But I don’t want to become a Synology expert. I want a simple storage device.
I think the root problem for these NAS vendors is that simply sticking drives and a RAID controller and an s by stack in a box is not a profitable business, so they try to differentiate by adding higher level features, and they are not very good at it.
I will keep futzing with it. But I believe it will end up with the final remedy for poor gadgets, put it in a burlap sack with a brick and sink it in the lake (metaphorically).