abnormal size of the on-disk file.

 Hi There, 
I have a small table  companies (< 300 rows)  and its on-disk size shows 
abnormal 
  
i check it as follows: 
 SELECT relname,relfilenode  from pg_class where relfilenode=30977;
  relname  | relfilenode
-----------+-------------
 companies |       30977
(1 row)
ls -lsh  30977
487M -rw-------    1 postgres postgres     486M Apr 26 20:27 30977
bash-2.03$
  
i created a new table with the same content 
tradein_clients=> CREATE  TABLE t_a as select * from companies;
SELECT
its on disk file is very small 
tradein_clients=> SELECT relname,relfilenode  from pg_class where relname='t_a';
 relname | relfilenode
---------+-------------
 t_a     |    87812505
(1 row)
bash-2.03$ ls -lsh  87812505
196k -rw-------    1 postgres postgres     192k Apr 26 20:27 87812505
bash-2.03$
Can anyone explain this .. 
does unvaccuming makes tables  x2000  times largers?? 
regds 
mallah.