This is possibly a feature of inserting an image inline into an email via by using Ctrl-C within Paint (or a similar art package) then going to Outlook and using Ctrl-V to get an inline image instead of actually just attaching the image via Explorer.
To show you what is happening I have created two emails, both using a 430kb image as a source. Attaching the email as an actual attachment give me a 444KB email, while the Ctrl-C, Ctrl-V method gets me a 2MB email, and very different results in terms of how the email actually looks as well.
I end up with two emails:
The "attached" 444KB email looks like this:
While the "inline" 2MB email has a picture in the email itself:
The most likely reason for this is that when you copy and paste an image directly into an email then Outlook must decide itself what to do with the image data and probably only compresses it using PNG compression which for large photo type images gives much worse compression. When you actually attach the image by either navigating to the file from the "Attach file" dialog or drag-and-dropping it from Explorer then you are attaching the image file (400KB) itself rather than the image data (2MB) being copied via the clipboard.
Windows Photo Viewer on Windows 7 seems to pass the filename to Outlook and I see the "attached" file size rather than the inline size.