Are you using a BES?
If so, you will have to manually download it from the server (when opening the attachment just 'retrieve' full contents). This happens so every blackberry is not hogging battery power and signal strength to download attachments that may not even be needed on the device.
In addition, on the blackberry server there is an attachment service that formats the document for viewing on the device. The end result often being much smaller than the original 10mb file. So to be perfectly vague, it all depens on the type of file and how compressed the blackberry attachment service can make it...(this all assuming you're using a BES)