Book ePub are not always sufficiently compressed

[benoit74]

In order to solve #288 and #235 and #222, we've decided to stop compressing (optimizing) ePubs on our own.

Recent runs and analysis done in #374 proved that optimization we were doing on ePubs was not that useless.

See for example book IDs 63630

	2023-08	2025-10
Size	265K	520K
URL	https://dev.library.kiwix.org/content/gutenberg_de_all_2023-08/Der%20Einzige%20auf%20der%20weiten%20Welt:%20Ein%20Menschenleben.63630.epub	https://browse.library.kiwix.org/content/gutenberg_de_all_2025-10/Der%20Einzige%20auf%20der%20weiten%20Welt:%20Ein%20Menschenleben.63630.epub

Or book ID 68838

	2023-08	2025-10
Size	438K	4.6M
URL	https://dev.library.kiwix.org/content/gutenberg_de_all_2023-08/Der%20Graf%20von%20Saint-Germain:%20Das%20Leben%20eines%20Alchimisten.68838.epub	https://browse.library.kiwix.org/content/gutenberg_de_all_2025-10/Der%20Graf%20von%20Saint-Germain:%20Das%20Leben%20eines%20Alchimisten.68838.epub

It is very important to note that many ePub of 2023-08 were missing all images (including the two examples above, due to #222) but it is not sufficient so far to explain all the file size increase.

I assume it would be safe to:

• first fix Book HTML images are not always sufficiently compressed #375
• adapt to optimize ePub images (while we could theoritically reuse the same images, not sure it is feasible, at least use some compression settings)
• confirm expected size difference (should save about 3G on Gutenberg DE)
• if size difference is not there, check what else could be optimized in ePub
• if not yet done, decide how to handle optimization cache invalidation (see Scraper takes ages to complete EN version and books are regularly updated #288)
• put optimization cache back in place

Remind that we've moved to ePub3 format, so the optimization logic is probably going to be different from what we used to have

[eshellman]

if you're using the PG epub3, they will be larger than epub2 for two reasons:

1. max cover size/image size is larger
2. some epub3 books contain audio files

[benoit74]

OK, but the books above have no audio, and I don't see why a larger cover image would 10x the ePub size. At least worth checking if just compressing images a bit more is not sufficient to gain significant size.