Library/Group Containers takes too much disk space

Finally I've got a reply from Mountain Duck support.

Here's a quote:

"The cache that you are referring to is the synchronization cache of Mountain Duck. The cache gets so big because it can't delete itself automatically.

You can delete the sync cache by deleting the folders Sync and Cache out of the application support folder. You can reach the application support folder by navigating to ~/Library/Group Containers/<hash>.duck/Library/Application Support/duck/.

Caution: Please make sure that all your files are successfully synchronized as all files that aren't successfully synchronized will be lost afterward.

Unfortunately, there isn't any way to limit the sync cache size. If you don't need the offline features of the Smart Synchronization mode you can switch to the Online mode which doesn't create the sync cache. You can switch to the Online mode either globally within the Mountain Duck preferences Sync tab or only for a specific bookmark within the bookmark configuration window of that bookmark."

So that clarifies my question and if anyone encounters the same issue I hope this thread could be helpful. Thanks peter_watt and dialabrain for your help and useful clues which helped me realise who was the culprit in this story.

Man, that's something! I'm using Mountain Duck to mount my NAS (somehow I thought that that directory is shared among many apps not related to just one), perhaps that's their cache. Maybe I should ask them what I can do with it! Cheers!

Have no faintest idea. Was wondering about it too. Perhaps some codename for the current version of MacOS or something like that?