SQL Server tempdb question
You should understand how SQL Server data storage works. It isn't like you are used to with client applications.

For performance and other reasons, SQL Server reserves blocks of space on the HD (in the MDF file) for the each database's use (including tempDB). That space as seen by the file size is still reserved even if it is not actively being used.

Whenever the database exceeds the allocated space (depending on the file growth settings), it may grow the amount of reserved space (never to give it back). So 30GB is either the initially reserved space or you did some huge operation at some point that caused it to grow really big.

Tips:
You can use sp_spaceUsed to see how much of that file is actually in use.
You can shrink the database back to size (if it isn't all used) with DBCC_SHRINKDATABASE
As tempting as it may be, don't turn on the DB's AutoShrink property.

As for the last modified date, here is what I found:   


  The modified dates change when SQL
  Server closes the files (SQL Server
  shut down or the database is
  detached), or when the file is grown
  (either automatically or manually). 
  All other times, SQL Server
  essentially bypasses the file system
  when performing the writes, and so the
  modified dates aren't updated.