Although 9MB is not huge, some internet service providers (Like Earthlink, AOL, etc...) block e-mail attachments larger than 5MB. Generally you would receive a "bounce back" message if this were the case- something that indicates a failure to send.

It might actually be an issue with the MIME type encoding from the PC to the Mac- and it simply doesnt show up because it is dealt with as extraneous garbage. This is the method for encoding the attachment, its not terribly important to understand- just technical information about how attachments are handled.

First: What E-Mail program are you using on the PC? What version? For example- Outlook 2003, Outlook XP, Outlook 2000, Outlook Express, Other?

What version of OS on the Mac side? Is it at least 10.x? What e-mail client are they using?