Medieval books were constructed of parchment, mainly sheep skin. If you take an average sheep and skin it and trim off the odd parts, like legs, etc., you get one gigantic sheet of parchment. When it is folded in half, it makes two pages of an old book. Fold it in half again and you get the size of an encyclopedia. In half again and you get the current hardback book size.
Next time you're squinting at your copy one of my books (you do have one, don't you), wishing the pages were just a bit bigger, blame the medievals for not having bigger sheep. Kind of reminds me about the width of railroad tracks and horses behinds, but that is for another day.