E-Mail with attachments (3,5 MB in Entourage. Why?
This isn't an Apple or Entourage issue, this is normal behaviour that happens when attachments are encoded to be sent with emails -- encoding the data to the email format makes the attachments roughly one-third heavier. 

This has nothing to do with the Apple filesystem. 

There is no way to stop this happening. If file size is a problem, consider uploading to an FTP server, or using a system like Box.net, YouSendIt, or DropBox to share the files -- these methods don't require encoding and will not increase the size.