Goto the URL opera://about, take note of the Opera version, the "install" path and the "profile" path and then close Opera.
In the install folder, you can delete all the older version folders if you want. Just don't delete the current version you're using.
In the profile folder, see what folder is using all that space. See if it's the "File System" folder (if present) or if it's the "IndexedDB" folder.