Answer
There was a localhost bypass in place on earlier builds which meant that messages sent through GMS Webmail were able to bypass the maximum message size setting.
This has now been changed and while messages originating from "localhost" (implying that they were generated on or by the server – this is the case for GMS Archive & Recovery logs or postmaster alerts) still bypass the maximum size check in order to facilitate certain server side functions such as archival backups to take place, there is a maximum message size check in GMS Webmail on "Send" so that if a message that is too large is attempting to be sent, it will be blocked. The message limit is not enforced on a "Save".
See Also:
Keywords:webmail maxmessize maximum message size