We currently have a 10MB message restriction at the GWIA.
I recently configured a 10MB restriction at the MTA.

Subsequently, we had some complaints from users that messages with
attachments weren't being delivered.
Using MTA Message Tracking the messages in question were flagged as
Cross checking the GWIA logs with the mail scanner logs, we noticed that
an 8MB message on the mail scanner was registering at over 13MB at
the GWIA.

Could someone please help me understand why this happens? I assume that
this is expected...

Any references to docs and TIDs would be greatly appreciated.

Many Thanks,