No the white space (or unused space) is the difference between the physical size of the database and the amount of that file that XC is actually using.
e.g. Physical DB is 1Gb, XC is only using 500Mb, white space would be 500Mb.
White space is created either by using mailbox manager to prune data older than a certain limit or by removing mailboxes and XC purging them after the deleted items / mailbox retention limit.
Just keep an eye on how 1221 changes over time. If it goes down to 0 then XC will extend the database file as required.