Hello,
Usually 450MB is normal....
However..incase you feel that thats very high as well....
Then, Check this :
If you go to Admin > Right Click on the Local Site > Properties > You could go to the Database Tab and change the database size as per your requirement... (now that stands for the Database size ....)
Since your Concern is the Virus Defintions Folder Size....
Go to Admin > Right Click on the Local Site > Properties > You could go to the Liveupdate Tab
Change the Diskspace Management for Download from 3 to 2 or lower (as per your requirement)
(by default it is 3days)