[Tuning] Getting file disk usage

Short and simple. Uses the DMV to tell you how much IO each file is using. What you think may be the case is not necessarily the case. And remember that backups count. I'd recommend doing 2+ samples during the day, and comparing. That'll help remove the backups from the end totals. Sample_MS is the number of milliseconds the computer has been up.

SELECT 
  master_files.NAME, 
  master_files.type_desc,
  master_files.physical_name, 
  vfs.sample_ms,
  vfs.num_of_bytes_written, 
  num_of_bytes_read, 
  size_on_disk_bytes/1024/1024/1024 AS Size_in_GB
FROM sys.dm_io_virtual_file_stats(null, null) vfs
INNER JOIN MASTER.sys.master_files 
 ON vfs.[file_id] = master_files.[file_id] 
 AND vfs.database_id = master_files.database_id
ORDER BY (num_of_bytes_read + num_of_bytes_written) desc