A common question I receive from customers is “What happens to the Exchange database when we delete or move mailboxes out of it?”
Answer: Moving the data out will create empty white space in the database. The DB won’t become smaller on disk, however it will be full of empty space so as new data is added it will fill the empty space and the database size on disk will only increase once all the empty space is filled. I know thats a complex answer, I didn’t design the DB : )
If you want to get the space back on disk, an offline defrag (eseutil) can be performed. It’s relatively low risk when performed on a healthy DB. I like to have backups before performing. I generally review the event logs to see the exch maintenance event which will show how much empty space is available in the DB and only run offline defrag if the amount is significant.