runaway backend process

Any help with this problem would be greatly appreciated.  After
executing the following query, the postgres backend server process that
is executing the query grows in size until it consumes well over 200 MB
and takes several minutes to finish:

select mlno from table where (area='01' and sub_area='01') or (area='01' and sub_area='02') or (area='01' and
sub_area='03')or (area='01' and sub_area='04') or (area='07' and sub_area='01') or (area='07' and sub_area='02') or
(area='07'and sub_area='03') or (area='08' and sub_area='01') or (area='08' and sub_area='03') or (area='09' and
sub_area='01')or (area='09' and sub_area='02') or (area='04') or (area='05') or (area='06') or (area='99');
 

A similar query with fewer terms does not cause an abnormal increase in
backend process size, and executes in under a second.  Does anyone have
any suggestions as to what might be causing this behavior?

Redhat 5.2
PostgreSQL 6.5.1
196MB RAM
database has been vacuum analyzed
table contains ~ 10000 rows

Peter Andrews