In the eventsDB there is a tabel called 'details'

Maybe you wondered what this table contains.
The detail table is used to store the extra fields of events that you see on the Detail tab when you click the magnifying glass on an event. These would include SNMP trap varbinds as well as any other fields you extract from events using event mapping rules.

There is one downside on this.
It seems this table is never cleaned or cleared up.

This table can grow in time and now at my side it's 170MB big.

Now I would suggest to add this in your /etc/cron.daily/zenoss

execute this a the user root:

printf '# This mysql query will clean the details table\necho "DELETE FROM events.detail WHERE evid NOT IN (SELECT evid FROM events.status UNION SELECT evid FROM events.history);" | mysql -uzenoss -pzenoss\n' >> /etc/cron.daily/zenoss

This will add a query to your cron daemon and will clean the database every day.