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Learning Python, 5th Ed: latest PDF release shrunk from 14.9M to 11M

I just downloaded the May 2, 2014 update for Learning Python, 5th Edition, and the pdf is 11M. The previous PDF release, Nov 19, 2013, of the same book was 14.9M.

The index for each release shows 1507 pages total in each release. Since the number of page are the same, what's missing in the latest release that shrunk its pdf so much?

Normally, I expect a release to grow, not shrink, let alone drastically. Was it just a matter of compressing images more? I'll continue to use the larger PDF until I hear an acceptable explanation for the PDF file size reduction.