This might be something that everybody thinks of as a well-known fact, but I've been searching around quite a bit without finding anything definitive. Maybe I just have the wrong idea of what's supposed to be going on.
I'm using maildir+ quotas. For arriving mail, it works as I expect: new lines are appended to maildirsize to reflect the size of the new messages. But I also expected that mail that is deleted (and expunged, yes totally gone from the disk drive, verified from the command line) would lead to some kind of update to maildirsize or maybe even a complete recalculation. But that does not happen on my system (dovecot 2.2.9 on a Ubuntu variant).
Now, I know that I can run "doveadm" from a cron job or whatever to have the maildirsize info recomputed. I also know that the maildirsize file will be recreated when it gets larger than 4k. That's all fine. But I don't see why I should have to have this out-of-band cron job, and I don't see why my users should have to wait if their maildirsize file doesn't tip over the magical 4k mark (which is a lot of individual messages being recorded).
Is it by design that dovecot doesn't update maildirsize immediately when messages are expunged? If yes, why?