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.