regardless of whether it's true, greenspun reported that it's what the publishers believe:

"It is an article of faith in the computer publishing that bigger books sell better. They take up more space on the shelf and readers with a tech problem find a bigger book comforting. Readers aren't really sure what is wrong with their computer and they only have a few minutes in the bookstore so they figure the thickest book is the most likely to contain the solution."

http://philip.greenspun.com/wtr/dead-trees/story