In previous versions of Exchange Server there was a process called "online maintenance". This process ran at night and one of the tasks was to an online defragmentation to do within the database. The task was that the empty spaces are grouped together resulting in more efficient use of the Mailbox Database.
Get-MailboxDatabase –Status | fl Name,AvailableNewMailboxSpace
If we were to do an offline defragmentation on the Mailbox Database the mailbox database would be approximately a 90MB smaller.
This is on my test Exchange 2010 server with only a few mailboxes. In normal production enjoinment this will be a lot more.
In Exchange 2010 you can you a online mailbox move. There fore you can
better create a new mailbox database en move all the mailboxes to the new mailbox database.
Then you have the same result with no down time.