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).
The whole sheepskin, flattened out and folded in half, is one common size. Fold it again, and it's another size. All of these sizes and dimensions are still being used by printing houses in the 21st century. The Kindle, for example, is the size of a sheepskin folded over three times. Pyrdum provides further examples and concludes:
Next time you're squinting at your mass-market copy of Dan Brown's latest wishing the pages were just a smidge roomier, blame the medievals for not having bigger sheep.
Link via Wired | Photo by Flickr user David Masters used under Creative Commons license