Database size versus table data size

I ran the below query that queries all tables in the database and the total size for reserved space is 17GB. The database size is 294GB. Why is there such a big difference in size. I would expect the database to be a little bigger but not 277GB bigger.

DECLARE @TableName VARCHAR(100)    --For storing values in the cursor
--Cursor to get the name of all user tables from the sysobjects listing
DECLARE tableCursor CURSOR FOR  
select [name] from dbo.sysobjects  where  OBJECTPROPERTY(id, N'IsUserTable') = 1 FOR READ ONLY
--A procedure level temp table to store the results
CREATE TABLE #TempTable (     tableName varchar(100),     numberofRows varchar(100),     reservedSize varchar(50),     dataSize varchar(50),     indexSize varchar(50),     unusedSize varchar(50) )
--Open the cursor
OPEN tableCursor
--Get the first table name from the cursor
FETCH NEXT FROM tableCursor INTO @TableName
--Loop until the cursor was not able to fetch
WHILE (@@Fetch_Status >= 0) BEGIN     
--Dump the results of the sp_spaceused query to the temp table     
INSERT  #TempTable         
EXEC sp_spaceused @TableName     
--Get the next table name     
FETCH NEXT FROM tableCursor INTO @TableName END
--Get rid of the cursor
CLOSE tableCursor
DEALLOCATE tableCursor
--Select all records so we can use the reults
SELECT *  FROM #TempTable
order by 2
--Final cleanup!
DROP TABLE #TempTable

Alan