Exchange mailbox size larger after move

I have an exchange 2010 sp1 box running on Windows server 2008.  The box is giving me some problems so my plan is to move exchange to a new server.

I built a new exchange 2010 sp3 box running on Windows server 2012 that is now (for the moment) co-existing with my older exchange 2010 box.  The users with large mailboxes were contacted and asked to delete unneeded mail prior to me moving the their box.  How much was deleted or if everyone followed directions i'm not sure, but I should have saved some space.  

Here is my question.  On the old exchange box my mailbox database is 420GB.  I've now moved around 90% of those mailboxes to my new exchange server the mailbox database there is 500+GB.  I'm not sure why the database is larger, I figured at the very least it would be around the same size.  Does anyone know why?