This is a slightly old topic, but it's one of the top Google results so I'd like to share my info here. Amazon AZW3/KF8 files are much smaller than EPUB/classic MOBI due to their recompression of images, as noted in the OP.
Here's some examples I pulled from a cookbook. Retail EPUB (62MB) vs. Retail AZW3 (14MB)
http://screenshotcomparison.com/comparison/159478
It's not bad at all for the space it saves, but it's worth noting if you're releasing something photography-related.