Lots of people are having trouble with the akonadi database, which obviously cant work error-free all the time. Numerous people do report a substantial reduction in database size after a rebuild. There should be a command (menu entry) which deletes the present data base and forces kontact to rebuild it.
Even in AppleMail the occasional rebuild of the database improves speed and reliability tremendously.
Thank you for the bug report.
As this report hasn't seen any changes in 5 years or more, we ask if you can please confirm that the issue still persists.
If this bug is no longer persisting or relevant please change the status to resolved.