Each member should represent the same number of voters (or electors), within a tolerance of +/- 10% of the average enrolment per member. This ensures that each vote has equal value, as far as practicable.
If two districts return the same number of members, but represent a different number of electors, then the votes (used to elect members) do not have equal value (when members cast their votes for or against Bills). If districts are knowingly designed in this manner it is termed mal-apportionment. Western Australia’s Legislative Council is guilty of this.