No Exmerg'ing going on here - I am aware of the single instance storage loss.
2003 SP2 to 2003 SP2, same org, same domain. Exchange System Manager -> Mailbox Store -> Exchange Tasks -> Move Mailbox.
Depends how you did it. If you exmerged, then you have lost single instance storage.
Moved mailboxes to a new server. Confirming this is normal and by how much should they grow?