Why is the new German parliament 12 % bigger?

Question

Before the German federal election in September 2017, the parliament (Bundestag) had 631 members; now it has 709 . How did this come about? Does it matter, or is it a second-order effect?

See also:
Electoral system of Germany: explains the voting system, but not the background of this increase
Is there an optimal size of a parliament: doesn't discuss the question, optimal for whom?

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Added, trying to clarify: why the huge increase ? Did the political parties just decide to give themselves more seats, a bit more power ? Voters were not asked.

Answer 1

The planned size of the Bundestag is 598 members: One directly elected member from each of the 299 electoral districts plus equally many members chosen from party lists in order to achieve a total allocation of seats that is proportional to the party votes (Zweitstimmen). As a simplified example, assume that party X got 10% of the party votes and that their candidates won in 13 districts; then party X will get about 60 seats in total (10% of 598), which will be filled with the 13 successful local candidates and 47 additional people from party X’s list.

However, this doesn’t always work out in practice because of something known as overhang seats: It is possible for a party to win more electoral districts than seats they are entitled to according to party votes. Say, party Y won 20% of the party votes (meaning they have a right to about 120 seats), but their local candidates won in 130 districts; now, even though party Y won’t get any additional members from their list, they have 10 seats too many, which can’t be taken away from them.

The Bundestag would have grown to 608 members in this example, and party Y would be overrepresented (130/608 = 21,4%, instead of the 20% they actually won). To avoid this effect and reestablish the proportional distribution, other parties will get additional seats, increasing the Bundestag size even more.

The problem is further exacerbated because seat allocation happens somewhat independently in each of the 16 states. Therefore, party Y may win overhang seats in one state, but party Z can at the same time also get overhang seats in another state.

Finally, the precise rules chosen to determine the number of additional seats aren’t mathematically sound and will often increase the size of Bundestag more than necessary, while still allowing the allocation of seats to fall out of proportion with the party votes.

In the 2017 election, the final size of Bundestag was determined by the results of CDU, which got a relatively low amount of party votes but still won a plurality in many electoral districts.

The increase in size is generally regarded as a problem because it makes parliamentary work more difficult. There are also practical issues in squeezing all the members into the main chamber and in finding office space for them.

However, for the individual member of parliament who wouldn’t have won a seat in a normal-sized Bundestag, the situation is obviously positive. This may contribute to a certain hesitation in addressing the underlying issues, which didn’t come as a surprise. Former president of the Bundestag, Norbert Lammert, has long argued for an overhaul of the election system, and there are many different proposals for how the enlargement effects could be avoided.

Answer 2

The German election system is such that the final proportion of parliamentary seats must match the country wide proportions of party-preference votes. Things are complicated because individual voters do not just have a general party-preference vote that will decide this overall make up of the parliament, but also vote for a personal representative to represent their local constituency.

There are 299 such local constituencies, which means 299 seats get filled directly no matter what the outcome of the party-preference vote. Then the remaining (planned) 299 seats get filled with party members in ways to achieve the right overall balance.

However, certain type of election outcomes cannot be made to add up in this way. The most recent election is such an example because one party (CDU/CSU) won 231 of the directly elected seats, while only receiving approximately 34.7% of the (eligible) party-preference votes across Germany. This means that keeping the directly elected 231 representatives of the CDU/CSU in the parliament, the total number of parliamentarians must be at least ~665 members (because 231 = 34.7% of 665).

The actual number of seats that the re-balancing procedure leads to is different (709) because some aspects of this re-balancing happens on a per-state basis. But the above is essentially the answer why the Parliament ends up with a different number of total seats after each election (even without changing the rules, which also has happened recently).

Answer 3

The electoral system is quite complex and in effect only specifies a minimum size for the parliament. The way this works is that about half of the member of parliaments are elected in a district using a first-past-the-post system but the overall share of seats allocated to each party still has to be roughly proportional to the share of the vote.

Crucially, the people elected in a district are guaranteed a seat in parliament no matter what, even if their party hasn't fared well in the global vote tally, which is why there have always been additional seats in the Bundestag. On top of that, a recent reform added yet another category of additional seats to deal with some paradoxes of the old system (under very specific conditions, a vote for a party could actually reduce the number of seats it would get).

In practice, the parliament is bigger when a party is strong enough to win districts by a plurality of the vote but has a (relatively) low share of the overall vote. Which is precisely what happened to the CDU in 2017, which obtained its lowest result ever (save for the very first election in 1949) and yet won most of the districts. It's also likely to happen again in the future as there are now 6 parties with a real shot at being represented in parliament (versus 3 in the 1960s and 1970s, 4 in the 1980s and then 5 after the reunification).

Finally, the number of MPs does not matter much per se but the increased number of parties represented in parliament creates more electoral oddities and makes traditional coalitions (like Schwarz-Gelb, i.e. CDU/CSU and FDP) less likely to command a majority. This in turns makes cabinet formation more difficult and forces parties to rely on the larger, sometimes uneasy coalitions that are usual in other parliamentary democracies with proportional representation.

(Technically, voters have two votes and can split them, voting for one party in the district-based first-past-the-post component of the election and for another one in the province-based proportional-representation component of the election but that does not matter all that much and I glossed over it for the purpose of this answer.)