He's using vulgar language to express mild disrespect. a) It's a big book, and b) he doesn't like it. Maybe it's a textbook he hates. Maybe it's a novel that's so long that it is heavy, and a nuisance to carry around. "Ass" here is a vulgar word for "buttocks." Literally, the expression would mean "has anyone seen a book with big buttocks," but of that doesn't make sense. "Ass" is pretty bad language, don't try to use it. The interesting thing is that "ass" is also an perfectly acceptable word for a donkey or jackass. Years ago people also used "ass" to mean "a foolish person," but nowadays people would probably say "jackass" in order to avoid confusion with the vulgar word.