This need not be a result of malapportionment. Even if all seats represent the same number of votes, the second placed party (in votes nationally) can win a majority of seats by efficient vote distribution. Winning seats narrowly and losing elsewhere by big margins is more efficient than winning seats by big margins and losing elsewhere narrowly. For a majority in seats, it is enough to win a plurality of votes in a majority of constituencies. Even with only two parties and equal constituencies, this means just over a quarter of the votes of the whole.