Books are as big as they are because medieval sheep were as big as they were:
[M]edieval books are no bigger or smaller than modern books, generally speaking. Gutenberg and the other early printers didn’t invent a whole new format for books, they just copied what people were already using.The question then becomes, I guess, why were medieval books the size they were? And the answer to that is simple: medieval books were the size they were because medieval sheep were the size they were. Remember, paper wasn’t the original medium for page-creation. Medieval books were constructed of parchment, which is a fancy word for sheep or goat skin (and primarily sheep skin, because there were a lot more of them around).