Mailbox bloating migrating from exchange 2000 > 2003

We are in the process of moving from exchange 2000 to exchange 2003. So far we’ve moved 15 mailboxes using the ‘move mailbox’ wizard. All of them have increased in size which we expected because of the loss of SIS. However 3 of them increased by about 1/3 to 1/2 that’s an increase of 200-300mb.

Is this kind of increase common? Is there any way of accurately predicting the increase in size due to losing SIS? Can this enormous increase be stopped?

Any help, thoughts or experience appreciated.