Looking at the first pair, it is obvious that these two pages are situated at the very beginning of the book. One reason is the extravagantly decorative cornice which contains a lot of complex ornaments. The second reason is the title of the book which normally appears one time at the beginning.
What is also interesting is the fact that the first couples of pages have some introductory sentences printed cursively.
The next pair shows the ground view of the Pantheon. It becomes clear that the page is extended and in case of closing the book the page needs to be folded. In other words, drawings seem to be important. Otherwise, the page size would not have been modified.
Sometimes, pages are filled with brief chapters which subsequently are accompanied by other drawings surrounded by a frame. In this case, it is the frame which catches one’s eye instead of the page size.