Part of the increasing in size is probably the datatype changes & all the new "stuff" in SQL 2k5. For instance, if you look under Security (in each DB) and check out SCHEMAS, a whole bunch of new schemas have been added for every last single login in the DB. Including NT logins. Might as well get rid of the NT login Schemas now. There's nothing you can do with them even though they're there.
Also, during the process of upgrading everything in the DB, SQL Server needs some extra space to make sure everything is what it should be as it goes through it's processing. You may be able to easily resolve some of your issue, though, by upgrading, your DB will already be 1.5 times larger than it was in 2000.
Run an Update Statistics and DBCC UpdateUsage on the DB, then a DBCC ShrinkFile / ShrinkDatabase. This might help you recover some space.
Catadmin - MCDBA, MCSA
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