Outlook 2010 HTML formatted email - Attachments vs Embedded email size

Today I composed an email in Outlook 2010 with some text plus two images. When I sent this email I cc'ed myself and was surprised to see that the email size was 11Mb! The question that immediately came up was why?



Here is the break down of what I found out:

1) Image file size
I checked the file size of both images. I was sure they were very small as I had photoshoped them and reduced the size. The total size was only 406kb.

File 1




File 2


2) Text quantity

There isn't more that 20 lines of text in the whole email (including the signature) so this can be excluded


I then repeated the test, sending one email with the only pictures embedded and the other with the same images but as attachments. The email with the attachments is almost smaller by 3.5M!




This probably has to do with how the embedded vs attached files are encoded, but what is it?