Notes on how to achieve small image size like Google Books. For background read:
Adam Langley, Dan S. Bloomberg, "Google Books: making the public domain universally accessible", Proc. SPIE 6500, Document Recognition and Retrieval XIV, 65000H (2007/01/29); doi: 10.1117/12.710609; http://dx.doi.org/10.1117/12.710609
A PDF of this document is in this repository.
Use jbig2enc to compress images (and also convert them to B&W). This library in turn depends on leptonica. Note that on macOS Sierra leptonica wouldn’t compile as it complained about a missing variable Z_DEFAULT_COMPRESSION. Installing zlib and adding:
#include "zlib.h"
to pngio.c fixed the problem. Note that you may also need to set file permissions on configure and config/install-sh for the installation to work properly.
Given the page image from BHL (34565786.jpeg)
jbig2 -s -S -p -v -O 34565786.png 34565786.jpeg
will create a smaller PNG. For darker images you may need to tweak the T parameter, e.g. use -T 100.
Consider opting and jpegoptim, see Optimise your pngs from the terminal in OSX.