The books in that picture are liturgical, and as such are meant to be seen by the whole choir (and other people involved in the service) from a distance. They needed to be big to be legible, and they needed to be thick to carry the various necessary permutations of the liturgy. One large book was easier and quicker to produce than a dozen small ones - you only had to copy everything once.

You can see the book opened in the picture contains musical notation.

As /u/SheepExplosion said, some are liturgical. Others were meant to be used in services, and were meant to be placed on a lectern and basically never moved, so they were made relatively large as well (the Gutenberg Bible is an example).

Some were also designed to be status symbols; see the Codex Gigas or "Devil's Bible," the largest extant medieval manuscript. (30 x 22 x 8.5 inches closed; when open it's larger than my kitchen table). It was said to contain "all the world's knowledge." That's not a book you're consulting every day.

Remember, books in this era are not simply vessels for the information they contain. (Well, they aren't ever simply vessels, but don't get me started). Manuscript books are fairly rare and valuable objects, and like any luxury good, they are designed to be attractive as well as useful. The more expensive the book, the more illumination could be expected, and bigger books could allow more creativity on the part of the scribe. (The Codex Gigas has full-page illuminated capital letters throughout).