Truncating a table or deleting data from a table does not release space from to the operating system from the database file(s), if does make that space available for re-use by the database. If you really need to release the space to the operating system you need to do a DBCC SHRINKFILE(). I wouldn't normally recommend that as the database grew to that size due to use and unless no new data is going to be added to the database, it will likely grow back to the size again.