A little late on this. If you're still interested, you can open the epub and extract the images folder. Run the files through an image editor like Irfan, which has bulk processing, to change the compression settings.
The resulting files will be smaller. Technically, there will be some loss of information, but when I compare the before and after images, I can't really tell the difference.
You can play with the settings until you get something you like and keep the original for later viewing as well.
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