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