[NiLuJe]

@topquark: Large amount of mostly empty (i.e., white) PNG images, which compresses well as a ZIP (ePub), but apparently not so much as huffdisc (I don't actually remember to which extent Calibre even tries to compress stuff, not much, IIRC).

Anyway, the better solution, a round of optipng on the lot of them, yields a tremendous amount of space saved (130MB -> 11MB). The default aggressive bitdepth reduction settings might not play well with every reader out there, but I'm not versed enough in PNG capabilities of various readers to actually tell.