It is proportional to the number of unread mails left in watched folders.  If you are subscribing to the folders and mark new mails as read on a regular basis, or let this agent do that with `mark_as_read`, it won't be a problem, I think.
The value typically looks like `{ "123456" => [567, 568] }` and once the listed mails (UID 567 & 568) are read they disappear in the next run.