No Single Instance Storage in Exchange 2010
With a big change is store schema, Exchange 2010 don’t support Single Instance Storage, which means if you send 2 MB attachment to 10 people with in the same database, it will take up 20 MB of your database size, in previous versions Exchange DB only had one instance of that 2MB attachment.
With the removal of Single Instance Storage from 2010, the exchange product team 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. So this limitation is not a very big problematic when you conceder the scalability options available in Exchange 2010.