I need to do more research on this, but I think you're catching IMAP unfairly in your dragnet here.  I would call this something like "bytesToDownloadForBodyDisplay".  For IMAP this is the sum of the bodyReps.  For POP3 this should indeed be everything, although it might be possible to simplify things by just putting all the download blame on a synthetic body part/the last body part.  That way we don't need to surface whether the message is from a POP account or an IMAP account.  (When we do unified inboxes, it won't be enough to just look at the folder/account.  It needs to be inferred from the header.)