Thanks for your replies.
Camponez suggests I run update.php[1] which I believe among other things dumps the objectcache table[2].
In the past I have only run update.php after an upgrade. But it ran on those occasions without incident. So I would feel fairly safe running it now, to see if that brings the database back down to size. If it does, I may run it monthly before each monthly backup so that the backups are not bigger than necessary.
Does update.php sound like a good first thing to try to brings the database back down to size?
[1] https://www.mediawiki.org/wiki/Manual:Update.php [2] https://www.mediawiki.org/wiki/Objectcache_table
Roger