A loyal MR reader writes:
Why are there so many well-padded books out there that really ought to be nice, long articles?
David Sucher has raised similar questions in the MR comments. The answer is simple: most people don’t read the books they buy. But they like the self-image generated by the book purchase decision, and they like to feel they are getting something for their money. Driven by market demand, book publishers demand a certain amount of heft and sometimes this means padding.
Yes there is a tendency toward shorter "books," some of which are called blogs. The price is lower. Another loyal MR reader once wrote in praise of MR: "if I wanted to read something longer I would read a book or something". Or not read, as the case may be.
Addendum: Note also that marketing expenditures are more or less constant, relative to the size of the book. Higher marketing expenditures (definitely the trend) thus spur higher-margin and typically larger books, as suggested by the Alchian and Allen theorem (why buy a big ad campaign for book which sells for a penny?). For those of us who actually read the books, as book choice goes up, the importance of marketing goes up, and the padding goes up as well.