How can the same email have two different sizes?

If you look at the size of an email in OWA and then preview the same email in Outlook you'll see that the message size might not be the same....? The reason of course is that Outlook is displaying the message via MAPI (and TNEF) and it's connection to either an .ost or a mailbox role server. OWA makes its connection via CAS which is displaying the streamed MIME size.  Obvious when you think about it really...