Readers enact their own resizing and compression though -- so the image you get will depend on both how the publisher created the ebook and then what the distributer/seller did with it. EBJ is one which seems to mostly offer you the image the publisher gave them for example, whereas kindle ebooks tend to go through substantial compression after they get handed to Amazon.
True, I found sokuyomi and niconico to be worst. kindle was the same until a while back, but dozen books i purchased last october to december seem to have drastically improved. I haven't been able to purchase anything from amazon since then, so I can't say what they're like now.
ebj, bookwalker, honto (to name a few) I've never really had that problem. though ebj seem to resize all their larger stuff to 1600px height. (e.g. most readers offer comic valkyrie at x1920px, but ebj's is x1600). maybe they keep paid books in their original resolution but only resize the free stuff. maybe someone bored can rip the sample of mikakunin v7 to check. still, very few publishers (i.e. ichijinsha) go beyond 1600px, so it's not really a problem. their lower res (mainly shueisha and older volumes) books are x1200.
I don't purchase much from ebj so i don't have enough samples to be 100% sure. I dislike how the ebj adds a white border to the one side of the page (i've even seem them crop a cover). i guess it's to fit their reader.
ebj are clearly the best for kodansha titles. I wouldn't say they are better for other publishers anymore, most seem to be pretty good these days.
Hm, the BookWalker software for the PC saves eBooks locally to the file system now (by default, AppData) as encrypted images, so anybody who has reverse engineering experience may be able to crack the decoding algorithm. Thought I'd put this out there.
The previous extractor for BW patched the log4net.dll file. it's has since doubled in size after the various updates (assuming because it's using some super encryption now)