Try querying dba_data_files for a more high-level picture of the amount of space you are using. Also look in dba_free space to see space that is *not* used within these datafiles.

YOu can look in dba_indexes to see info similar to what you found in dba_tables. Another place to look is dba_segments.

Then there is also dbms_space.unused_space. Check the
doc for how to use it...

-John