too much rollback segment usage

too much rollback segment usage

I was doing
SQL> desc test
Name Null? Type

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

---
DASET_ID NOT NULL NUMBER(10)
SECT_ID NOT NULL NUMBER(10)

begin
for i in 1 .. 10000000 loop
insert into test values (i,1);
end loop;
commit;
end;
/
PL/SQL procedure successfully completed.

After it finish, the highwater mark of table is only 150M, but during the
inserts, rollback segments grow up to 680M, why do we need such a big
rollback segments for such a small table?   According to some OCP book,
insert only store new rowid into the rollback segments.

Thanks for your help.

by the way, there is no indexes on the table