Exchange 5.5 Single Instance Message

I would like more information on Single Instance Message. Specifically how it relates to the amount of storage used. If a 10mb file is sent to 100 recipients are server resources decreased by 1000mb or 10mb + pointer if all recipients retain the mail. The following comment from XADM: Single Instance Storage Ratio is Low (Q198673) is not clear ? "There is a common misconception that the primary benefit of single instance storage is that it greatly reduces the storage space requirements for user data on a mail server." So what is the relation between Single Instance Message, server resource space, and individual mailbox space.