@leocg I removed website data one by one and checked if there was a change of folder size. After deleting multiple websites and checking actual change of disk space, I assume it's just a minor bug. For some reasons Opera does not count the occupied space correctly.

Thanks for your help!

@leocg Thank you for clarifying.
The information found under opera://settings/cookies/detail?site=... relate to the data shown with opera://settings/content/all.
Unfortunately, opera://settings/cookies/detail?site=... does provide limited insight into the actual content of the saved data. What I see is that the majority of the data is located at in the Cache Storage and not in Local storage or Database Storage.
But that's even more surprising because ~/.cache/opera is just 5.4MB in size and not 6.9 GB. Or is there some kind of general cache I don't know about? My /tmp and /var/cache folder are far smaller in size.

Maybe it's a bug? Opera is showing a bigger cache size than disk space is actually used.

@leocg Thank you for your help.
As far as I understand, cookies are saved in the profile folder, which is ~/.config/opera according to "about Opera". This folder I checked, it only contains 1.4 GB of data.
What do you mean with "local storage"?
If there are more folders where Opera saves data, despite ~/.config/opera ~/.cache/opera, then I do not know them. I'd be happy to learn about other directories to check.
I coul not find any related folders using Linux command find / -iname opera.

Opera Version: 77.0.4054.146
Ubuntu 18.04

Hello, under opera://settings/content/all Opera claims that:

Total storage used by sites: 6.9 GB

I wondered why so much data was saved. According to Opera, one website alone uses 2.5 GB. I tried to find out what was saved but the disk space did not match Operas numbers.

du -d 0 -h ~/.config/opera ~/.cache/opera

1,4G	~/.config/opera
5,4M	~/cache/opera

Maybe I looked at the wrong place?
Where can I find the stored data?

Thanks & best