Opera writes session.* files to disk each ~10 seconds. They are SQLite databases and can grow. I've had a 4 MB session.db once. You can imagine how this would affect my SSD, if it wasn't on a RAM disk.
To reduce writing onto SSD, move session.* files to a RAM disk, replacing them in the original profile with NTFS symlinks.
To reduce the file sizes if they've grown too big:
If you want to keep session content (remember which pages you have open), use the command,
sqlite3 session.db "VACUUM;"
(get sqlite3 from http://www.sqlite.org/download.html)
Or, if you don't care about sessions, just delete the files. Opera will re-create them.