Unless you run offline defragmentation by using eseutil /d command the Exchange database file will never shrink. It will free up space within its own database so that the space can be over written. However, it will always use the space on the disk until you run offline defragmentation. If you want to view the whitespace in an Exchange database file you can by looking in the application log for Event ID 1221.
*Before ever running an offline defrag you should always make a copy of the database first and run the defrag on that copy. That way if anything goes wrong you still have the original database file. Exchange databases are very vulnerable and doing this extra step can save you tons of headaches.