We went through an exercise deleting 10's of million of custom records back when we were told our total storage was over allocation. It didn't change the size in MB at all. We then found out that the total storage size was only recalculated once a week, on Sunday, so we waited until the following Monday and checked again, and there was still no change at all. We then reported that back to NetSuite saying we couldn't verify the size of each record type before and after, and therefore couldn't even independently verify if their total storage size was accurate, or reflected changes we made. The bottom line is that you should be in communication with your account rep before making any drastic changes, and ask what the expectations are and how you can meet them on your end.