When you see a merger between two giants in a declining industry, it can look like the financial version of a couple having a baby to save a marriage. At least that was my thought when Random House and Penguin, two of the world’s six largest publishers, announced that they were coming together last month. Ever since Amazon began ripping apart the book business, the largest houses have been looking for a way to fight back. If this merger is any indication, they have chosen an old-fashioned strategy: Size.