Exchange 2013 mailbox size increase will occur when it is migrated from a previous version of Exchange. This behavior is document in the Exchange 2013 release notes. The logic for this is explained extremely well on Tony Redmond’s Exchange Unwashed Blog where he states that it is a “store tax”. The Exchange release notes describe the situation:
When you move a mailbox from a previous version of Exchange to Exchange 2013, the mailbox size reported may increase 30 percent to 40 percent. Disk space used by the mailbox database has not increased, only the attribution of space used by each mailbox has increased. The increase in mailbox size is due to the inclusion of all item properties into quota calculations, providing a more accurate computation of space consumed by items within their mailbox. This increase may cause some users to exceed their mailbox size quotas when their mailbox is moved to Exchange 2013.
The bottom line is that Exchange 2013 accurately identifies all of the information about mailbox items and counts it towards the mailbox quota. Expect a 30 to 40 percent increase in mailbox size when moving to Exchange 2013! You will certainly want to do some forecasting well ahead of any migrations as well as automating a quota increase for your users.