The store schema in 2010 has been given a big change and as a result of the new store architecture, Exchange 2010 no longer supports Single Instance Storage (SIS). This means that when you send a 1mb attachment to 50 users within the same database, the size of the database will grow by 50mb. In previous versions of exchange, the attachment is only stored once and all recipients get a pointer to the actual attachment from which it opens. Hence the database only increases in size by 1mb.
With the removal of SIS from 2010, the exchange product group estimates an increase in database size of upto 20%. This has to be considered when designing a 2010 environment. As the Extensible Storage Engine (ESE) is optimized for handling large (10Gb+) mailboxes in 2010, the 20% increase in storage is not a big deal considering that you don’t need expensive RAID solutions if you have a DAG with 3+ copies of your database. Plus the hard drives are getting bigger and cheaper these days!