@sostberg You can reduce the database size by deleting old rows from the user history table. You will not then have the complete history for some subscribers but that usually isn’t going to be a problem.
phplist doesn’t provide a way to do this so you will have to run a query in phpmyadmin. Depending on how far back the records go, perhaps deleting records that are more than 12 months old will reclaim some space. You can see how many records will be deleted with this query
select count(*) from phplist_user_user_history where date < current_date() - interval 12 month
Then to do the actual deletion
delete from phplist_user_user_history
where date < current_date() - interval 12 month