So let's see how this works out:
// clean up the database
$a->config['system']['dbclean'] = true;
$a->config['system']['dbclean-expire-days'] = 600;
This setting should delete all public posts from other server with no user on my server owning them after 90 (??) days, that's the first setting.
The second setting should expire all external posts owned by some user locally.
I think #Friendica
picked up to work on the cleanup and expiry this last hours, I can see 2000 open task in the queue.
I did this because the database was getting bigger and bigger an I heard other server do this as well, so when I read that my database is bigger than other public server database I started thinking.