It appears as if there is a bug with Folder Auto-Clean options when you have it set to trigger based on size of the folder. It seems similar to the issue that was confirmed as a bug in this older thread -

https://portal.smartertools.com/community/a91416/folder-autoclean.aspx#121542

We are using Build 7068, and Folder Auto-Clean options are not working when you have them set to clean up folders by size. I tried setting various different rules on multiple folders. The test folder I used had 19.6 MB of email in it, and I tried the following rules -

When size is over 5 MB, reduce to 1 MB

When size is over 1 MB, reduce to 0 MB

When size is over 0 MB, reduce to 0 MB

None of these appear to trigger. On this same folder I setup rules to trigger by age, and placed some older email in them and they were cleaned up in no time at all, and autoCleanFolders.log showed them being removed. This is happening on multiple domains/email accounts. "Override auto clean settings" is enabled.