Smaller data types use same disk space

Hi,
I've created two tables labeled "Big" and "Small" that both store the same 10 million rows of data using 493MB and
487MBof disk space respectively. The difference is that the "Big" table uses data types that take up more space
(integerrather than smallint, float rather than real, etc). The "Big" table should need about 27 bytes/row versus 16
bytes/rowfor the "Small" table, indicating to me that the "Big" table should be 70% bigger in actual disk size. In
reality,it's only 1% bigger or 6MB (after clustering, vacuuming and analyzing). Why is this? Shouldn't the "Small"
tablebe about 110MB smaller (11 bytes for 10 million rows)? I'm estimating table size with \d+ 

Thanks, Robert

         Table "Big"
  Column  |       Type       | Bytes
----------+------------------+-----------
 rmid     | integer          | 4
 date     | date             | 4
 rmfactor | text             | 7 (about 3 characters/cell)
 id       | integer          | 4
 value    | double precision | 8
---------------------------------
 Total Bytes/Row               27
 Rows                          10M
 Actual Size                   493MB


    Table "Small"
 Column |   Type   | Bytes
--------+----------+-----------
 rmid   | smallint | 2
 date   | date     | 4
 rmfid  | smallint | 2 (rmfid is a smallint index into the rmfactor table)
 id     | integer  | 4
 value  | real     | 4
---------------------------------
 Total Bytes/Row     16
 Rows                10M
 Actual Size         487MB