Well from our perspective with The End, we chose the small size primarily to support the feel of our setting. Since the setting is a post-biblical apocalypse, The End is designed to resemble a bible or preacher's book- which we think its faux leather cove and gold foil stamping does quite well.
I must admit, it is easier to manipulate as well. From a publisher's perspective, the book trade finds it easier to work with also.