Exchange 2010 - mailbox True Size

Why does the Exchange mailbox show different sizes when I run 2 different commands on the same mailbox?

For example I'm reorganizing the mailboxes and databases and I run this command below and it shows the mailbox size 4.357GB

>Get-MailboxStatistics auser | ft DisplayName,Database,TotalItemSize,ItemCount,storagelimitstatus


DisplayName                        Database                           TotalItemSize                                               ItemCount
-----------                        --------                           -------------                                               ---------
auser                     Legal Database                     4.357 GB (4,678,566,166 bytes)                                  57605

So being that it's 4.357 GB I moved it to the 5 GB limit database as shown below, but the size comes up as 5.759 GB.

>New-MoveRequest -Identity auser@domain.com -TargetDatabase "Mailbox 01 Limit Database"
DisplayName                                 Status                    TotalMailboxSize                            TotalArchiveSize
-----------                                 ------                    ----------------                            ----------------
auser                                             Queued                    5.759 GB (6,183,689,279 bytes)

So now the user is already over the limit before even being fully copied over to the new database. Which number is true?