Both of these early printed books could have been printed on the same size sheets of paper

When you look at an old printed book you’re not really looking at a single book but a series of smaller ‘booklets’ joined together. Books weren’t printed one page at a time but on large sheets of paper subsequently folded into booklets (called gatherings). Therefore pages aren’t the building blocks of early printed books; sheets are. Whether it’s a teeny wee pocket book or a heavy big lectern book, it may well have started out being printed on on the same size sheet of paper.¹

A big book, like a lectern Bible, will often contain a series of four page booklets joined together, each booklet formed from two pages printed on either side of a sheet of paper which has then been folded once (books made in this way are described as folio format).²