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This article proposes a comparative study of the politics that aim to reduce the number of MPs in France, Portugal and United Kingdom. It shows how rationality and mathematization of this issue are unsatisfying, since they are unable to provide adequate interpretation of the evolution of MPs number. The number in itself is less important for political actors than the functions and consequences they ascribe to the downsizing. Based on a study of the arguments in favour of the downsizing or the status quo, this article puts forward an approach to electoral reform through ideas. The construction of two référentiels, “decisionist” and “representativist”, reveals that the question of the number of MPs lies within a larger concern, with regard to the utility and the functions of political institutions. This article contributes to the place of ideas in public policy analysis.