PostgreSQL group sort

use these sqls:

CREATE TABLE big(id serial, name text);
CREATE TABLE sm(id serial, big_id integer, name text);

INSERT into big (id, name) VALUES (1, 'big1');
INSERT into big (id, name) VALUES (2, 'big2');

INSERT into sm(id, big_id, name)VALUES (2, 1, 'sm1');
INSERT into sm(id, big_id, name)VALUES (1, 1, 'sm2');
INSERT into sm(id, big_id, name)VALUES (3, 1, 'sm3');
INSERT into sm(id, big_id, name)VALUES (6, 2, 'sm4');
INSERT into sm(id, big_id, name)VALUES (4, 2, 'sm5');
INSERT into sm(id, big_id, name)VALUES (4, 2, 'sm6');

--------------------------------

run :
SELECT b.id, array_accum(s.id), array_accum(s.name)from big b, sm s where b.id = s.big_id group by b.id;
(ps: array_accum is aggregate inhttp://www.postgresql.org/docs/9.0/static/xaggr.html)

id | array_accum | array_accum
----+-------------+---------------
1 | {2,1,3} | {sm1,sm2,sm3}
2 | {6,4,4} | {sm4,sm5,sm6}
(2 rows)

the excepted result:
id | array_accum | array_accum
----+-------------+---------------
1 | {1,2,3} | {sm1,sm2,sm3}
2 | {4,5,6} | {sm4,sm5,sm6}
(2 rows)

It is a group sort ?