Hi,
When creating tablespace in Oracle. The rdbms allocates the size of the tablespace given in disk space.
When creating a table in the segment of the tablespace , the rdbms allocates an extent inside the tablespace for the specific table. This extent will handle all of the table's needs (insert,delete except update) and the rdbms will allocate another extent if needed.
If you delete data from the table, you will not reduce the size of the datafile file for a certain tablespace.
if you want to reduce disk space simply drop a tablespace.