Germany’s parliament is bursting at the seams. It may get bigger

NO VOTING SYSTEM is flawless, as any political-science student can tell you. Britain’s first-past-the-post method can give a thumping majority to a party that wins far less than half the vote. Ultra-proportional systems, as in the Netherlands, lead to fragmented chambers full of fringe parties, with no local links, devoted to animal rights or the elderly. Germany’s “mixed-member proportional” system is supposed to offer the best of both worlds. Unfortunately its size has begun to matter.

Of the 598 seats Germany’s electoral law reserves to the Bundestag (the upper-house Bundesrat comprises state politicians), half are for directly elected constituency MPs, and the rest are for candidates taken from party lists along proportional lines. At general elections Germans therefore cast two votes: one for a local MP, and one for a party. The second vote determines the relative strength of parties in parliament. If some win more constituency seats than their share of that vote would entitle them to, to preserve proportionality others are compensated with party-list seats. This means the size of the Bundestag can go only one way: up.

The problem has grown acute as Germany’s party system has fragmented. For big parties, the gap between their number of constituency seats and their shrinking overall vote share has grown, meaning...