Parliamentary Voting System and Constituencies Bill

I thank the right hon. Gentleman for giving way, but why has he settled on a figure that is different from the one proposed by him in the Liberal Democrat manifesto—which was to reduce the Chamber by 150, I believe—or by the Conservatives, who sought to reduce the number to 585? Has the figure of 600 been settled on because going any further in the downward direction would affect Tory and Liberal Democrat seats rather than just Labour ones, as proposed?