The bigger the book, the longer it takes to finish writing and formatting it, the more it costs to produce and purchase, the more errors it will have, and maybe the fewer people who will buy it.
I never go to movies that are longer than two hours, because I know the movie will become a $12 nap. I am similarly reluctant to buy books with more than about 350 pages, because I doubt they will keep me interested.
In an online forum for authors, a newbie recently discussed some fine points about his debut novel -- which will have more than 800 pages.
It will be extremely difficult to persuade people to buy a huge and expensive book written by someone they've never heard of. Maybe that book should become three books, or should be drastically cut. Almost any page can sacrifice a sentence or two without suffering. Most sentences can shed a word or two, and no reader will miss them.