Understanding Single Instance Message Storage
To help control the size of the mailbox stores, Exchange supports single instance message storage. This means that when a message is sent to more than one mailbox in the same store, only one instance of the message is stored, in one mailbox. The other mailboxes contain pointers to the stored message.
If the message is sent to mailboxes in a different mailbox store, the message is written one time to each mailbox store.
Single instance storage may not be maintained when a mailbox that contains a message is moved to a server that contains a mailbox store with the same message.
To maximize single instance message storage, put similar users in the same mailbox store, such as users in the same department who use Reply All or users who send large attachments to one another frequently.