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Why do proportional representation systems use multimember constituencies?

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Map of Canada displaying the results of the 2008 federal election by province and territory, with regions shaded to indicate the leading party's share of the popular vote: blue for Conservative, red for Liberal, light blue for Bloc Quebecois, and orange for New Democratic Party. Each area is labeled with the number of seats won by each party, and a legend at the bottom summarizes national seat totals for each party. — election Canada's 2008 federal election results.

Proportional representation systems use multimember constituencies to ensure that the distribution of legislative seats broadly reflects the distribution of the popular vote among competing political parties.12

Unlike plurality or majority systems, which typically use single-member districts that reward strong parties and penalize smaller ones, multimember constituencies allow multiple candidates to be elected from a single district. This structure enables parties that do not hold a majority or plurality of the total vote to still secure legislative representation, thereby ensuring that minority groups receive a measure of representation proportionate to their electoral support.231

The degree of proportionality in these systems is heavily influenced by the "district magnitude," which refers to the number of candidates an individual constituency elects. Generally, the larger the number of seats per electoral district, the more proportional the final outcome will be. By utilizing these larger districts, proportional representation seeks to create a representative body that mirrors the full diversity of opinion within the electorate.12