Since SharePoint Server 2007 (and WSS 3.0), SharePoint has something called Gradual Site Deletion. The reason is that directly deleting huge sites from the Content Database would block the database access (and thereby functionality of SharePoint sites that are located in the same Content Database) for quite some time, as potentially millions of table rows will have to be removed.
So what SharePoint does when "deleting" a site (collection) using the Central Administration (or PowerShell with the right switches) is to just remove the site from the available sites in the Configuration Database and to add an entry in the Configuration Database in some sort of queue of sites to delete.
A TimerJob called Gradual Site Delete is running daily (if not configured otherwise) to go through that queue and to delete the marked sites in small batches of row deletions, which is not as heavy on the load as an instant deletion would be.
After the site is completely deleted by the TimerJob, the space should also be shown in SQL Manager for the Content Database as being available/unused.