Help:Reduce size of the database
From MozillaWiki
It may be possible to reduce the size of the database.
Permanently remove the history of deleted pages
To permanently remove the history of deleted pages, first delete the pages using the normal page deletion feature.
Then clear out the archive table, where deleted revisions are stored, at the database level:
DELETE FROM archive;
Of course, it will then no longer be possible to view and restore deleted pages and their history.