A paperback will never seek that most comfortable size for the simple reason that a 400 page book, might now be 800. (goodereader.com)
The bird should spend large amounts of supervised time outside of its cage, so the cage needs to be of comfortable size when the bird is in it. (rescuethebirds.org)
I will confess that these gave me a good amount of muffin - top at the beginning, but have since stretched out to a very comfortable size. (bluecollarredlipstick.com)