Originally Posted by Ripplinger
That's just very poor formatting by the publisher. With minimal knowledge anyone can run the cover through a graphic program and compress it correctly so you don't see any difference in the quality. There's no reason a cover for an epub needs to be over 100kb in size, and most of mine are well below that.
Most other small images for fancy chapter headings or dividers are usually under 10kb each. Again, if done properly to not waste space for no reason.
I've seen ebooks where they use the same vignette for each chapter heading and a separate image has been included for every chapter. The same for ebooks that use a triple bullet to separate segments and used a separate image file for each instance.
Just plain sloppy work in my not so humble opinion.