How to find out which object is taking space?
It appears that your database data file has empty space. 

If you know for sure that the data file will not grow out again, you could shrink the file. 

If you have to shrink, the cleanest way is doing this with TRUNCATEONLY so you don't move any data pages and only release the space back to the OS if it is possible.

You can get the data file(s) with this query, change the databasename to your database.

SELECT [name]
FROM sys.master_files
WHERE database_id = db_id('DatabaseName')
AND type_desc = 'ROWS';


Afterwards you could try shrinking the data page until 100GB without any movement.

Use [DatabaseName]
GO
DBCC SHRINKFILE ('name',102400 ,TRUNCATEONLY);
-- try to shrink until 100GB, without moving data pages


You can read up on shrinking date files and...


why it sucks here
why it's bad for performance here