Outlook 2011 blows up attachments

One of my users presented me with an interesting situation today. When attaching a document to an email in Outlook 2011, the file size increases significantly. The file he was attempting to send was a spreadsheet that was 8.8MB, well under our 10MB file size limit. He even ran it through WinZip for Mac to compress it further down to 8.3MB. When he attaches it to his email in Outlook, the file size balloons to 11.3MB, over the message cap.

Can someone give me some insight as to what's happening here? He's spending money on FedEx to ship documents because he can't get them to go through our mail server.

Thanks!