The trolleys haven't been around that long, did they have the larger books on them in the beginning?
If so, it could be that they found that people aren't as likely to take a book in passing but might be inclined to grab a brochure....not as much of a commitment. Also the brochures are lighter for the person wielding the cart around and they are more to the point and specific and people may be more inclined to read them at a glance rather than put them aside as one might do with a book. They could be saving the books for later if and when they are truly interested.