When performing mailbox moves sometimes they complete with warnings. When you interrogate the move log you may find that the move was successful but the request failed to clean up the source database.

Microsoft say they resolved this in Microsoft Exchange 2010 SP3 but I have seen this happen in later releases SP3 CU5 and 2013.

To manually remove these disconnected mailboxes you need to use EMS. First find the disconnected mailbox in the source database

Get-MailboxStatistics -Database <source database name> | Where { $_.DisconnectReason -eq "SoftDeleted" } | fl DisplayName, MailboxGUID

Then use the following command to remove them

Remove-StoreMailbox –Database <source database name> –Identity <user mailbox GUID> –MailboxState Softdeleted

You will need to wait for the maintenance routine to run on the database before the white space is reclaimed