In Exchange 2010 you can see how big your mailbox databases are, and how much white space they have, by running the following command in the Exchange Management Shell.
[PS] C:>Get-MailboxDatabase -Status | ft name,databasesize,availablenewmailboxspace -auto
In screen-cap database AB is 64.76 GB in size but has 11.97 MB white space due to mail deleted. This is a nice clean server post migration. As time goes on the white space will grow and the need to defrag will be required to free up unused space. — this I will go into at in another post.