Weren't paperbacks originally made that size because it was convenient to
carry around? They were even called pocket books once right because
they were pocket sized?
Yep. Pocket editions were smaller than the traditional Post 8vo,
usually taken from a Small Foolscap sheet (13.25"x16.5") which gave a
trim size of 4"x6".
The standard paperback is a little larger at 4.25"x7", largely because
the stop-cylinder press used by the big book publishers was capable of
a much larger sheet, so it could be machined from Quad Crown 32mo.
Even though everything is web offset now, the size has been fixed
since the early 1920s, and no-one dares change it because there are
too many bookcases made to take paperbacks (true :-)
In the USA, two uncut sizes were still used for sheet-fed work until
recently, 30.5"x41" and 41"x61". Both give a trim size of 5"x7.325"
max, but the direction of the grain is different, which accounts for
the funny segmentation found in some US paperbacks, where the
signatures sometimes curled against each other in opposite directions.
Just thought you'd like to know :-)
///Peter, purveyor of typographic inconsequentialities to HTML-WG