<p>Since 2002, the regular size of the German Bundestag has been 598 seats. However, due to overhang and compensatory mandates, the Bundestag was enlarged to 709 seats after the 2017 election and to 736 seats after the 2021 election. This made the Bundestag the largest parliament of any democratically governed nation state in the world. To address this issue, the Bundestag passed a reform of the electoral law on March 17, 2023. This article explains the mechanics behind the significant increase in seats in 2021. Furthermore, it analyzes the conditions that must be met for a significant increase to occur. Specifically, it examines the relationship between the increase in surplus seats that triggered the expansion and the change in the party system’s structure. This relationship is demonstrated by developing a simple formal model. The German electoral system is a system of personalized proportional representation. However, as shown, some essential and desirable normative requirements that such a system should fulfill are mutually incompatible. Against this background and considering the history of the reform, it is demonstrated how the chosen reform option developed.</p>