I'm going to guess that the pressure to fold the Times' Sunday Book Review into a cheaper, thinner Saturday tabloid comes from the Chicagoans in temporary residence on Spring Street. They have some experience with this sort of move. From Michael Miner's "Hot Type" column in the Chicago Reader of Jan. 20, 2006:
The Tribune's books section carried an announcement last Sunday that as of January 22 it will become a tabloid. The unstated reason is to save money by cutting the amount of newsprint required to produce the section. It's one of only five freestanding book sections in American newspapers, and the Tribune, with a well-deserved reputation to protect as a friend of literature, wants to keep it. But it doesn't attract enough advertising to pay for itself.
The Tribune had intended to announce in December that in January the section would move from the Sunday to the Saturday paper. This was an interesting idea: while the Sunday paper is too enormous for its own good, the Saturday paper is too thin. But the big reason the change appealed to the Tribune is that Saturday's press run is some 400,000 copies smaller than Sunday's. The annual savings in newsprint alone would reach half a million dollars. At the last moment the Tribune remembered that its contracts with its distributors would require it to pay them extra to stuff another section into the Saturday paper. The switch wouldn't be worth it.
Books editor Elizabeth Taylor wasn't excited about moving to Saturday, but she says she's happy to go tabloid, even though she'll wind up with less space. Readers also prefer a tabloid, she tells me: "It feels more bookish." She hopes to find new advertisers. The traditional advertisers, the publishing houses, won't pay the freight, but she'd like to think colleges and cultural institutions would enjoy the company of literature. Even the occasional movie ad for a literary adaptation wouldn't be out of place.