The Size of Our Books Was Determined By The Size of the Average Sheep in the Middle Ages

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).