why the DB file size does not reduce when 'delete' the data in DB?

I had deleted a very large number of records out of my SQL table in order to
decrease the harddisk space.  But after I use command 'ls -l
/usr/local/pgsql/data/base/', it is found that the size of concerning files
do not reduce due to the effect of 'delete' SQL command.  What should I do
if I would like to decrease the harddisk space?

I am looking forward to your response.  Thank you very much for any help.
-
Jaruwan