Proportional representation (PR) electoral systems are supposed to ensure the representation of interests with small or geographically inefficiently distributed voter bases. Yet, in reality, most PR systems consist of a large number of districts that vary strongly in size and some have surprisingly low magnitude. Research shows that such differences matter for partisan outcomes and the effective proportionality of PR systems. We argue that the design of electoral districts in newly adopted PR systems is systematically linked to electoral geography and partisan politics. If parties with concentrated voter bases influence the design of the new electoral system, we expect to observe the creation of PR systems with a significant number of low magnitude districts. In general, parties involved in designing districts benefit from electoral disproportionalities under the new PR rules. Empirically, we use newly collected data for electoral reforms in ten European countries at the turn to the 20th century.