performance and number of selected columns

Hi,

say I have three tables a,b,c with lots of columns (say 10 for each
table). Using psql command line, why does

SELECT a.*,b.*,c.* FROM a,b,c

takes much more longer (in my specifc case 3 times) than only
selecting one column like

SELECT a.oid, b.oid, c.oid FROM a,b,c.

This is on 7.0. Can I play with buffer sizes? Indexes exist on most
of the columns. Columns in the second line can be arbitrary.

Dirk