I have a few users mailbox’s where the database properties say a different size than the total amount of documents indicate.
An example is: within the database properties the total size is 166Mb but when we do a total count of the size column when I have a SELECT @All view, the size is 95Mb. We are even using Single Copy Template so that negates the size of the template.
View and folder indexes take up space, and space that had been taken up by documents that have been deleted will not be released until the database is compacted.