For years, a few people noticed something that didn’t make sense. They would hit their quota limit, get “mailbox full” errors, and check their disk usage only to find plenty of room left. Most users never ran into it, but those who did were understandably confused.
The reason came down to how Dovecot measures mailbox size. By default, it doesn’t look at what’s actually stored on disk. It looks at the declared message size, meaning the size reported when the email was sent. That number is usually larger than the space the message ends up taking on disk, because attachments are base64-encoded during transmission, which inflates their size.
So, Dovecot just stopped lying about it.