Doesn't PostgreSQL clean data in data file after delete records form table?

Hi,
we are using PostgreSQL 9.3.6 version and observe data file size is not
decreased after we deleted records from the table. It looks quite abnormal.
Is it as PostreSQL designed?

DBTestPostgres=# select count (*) from test_data ;
0

# ls -l
total 788932
.............
-rw------- 1 _nokfssystestpostgres root *801067008* Mar 23 17:51 32768
-rw------- 1 _nokfssystestpostgres root 221184 Mar 23 17:47 32768_fsm
-rw------- 1 _nokfssystestpostgres root 0 Mar 23 17:44 32768_vm
-rw------- 1 _nokfssystestpostgres root 0 Mar 23 17:43 32771
-rw------- 1 _nokfssystestpostgres root 8192 Mar 23 17:43 32773
-rw------- 1 _nokfssystestpostgres root 512 Mar 23 13:57
pg_filenode.map
-rw------- 1 _nokfssystestpostgres root 116404 Mar 23 17:19
pg_internal.init
-rw------- 1 _nokfssystestpostgres root 4 Mar 23 13:57 PG_VERSION

Thanks!

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