Size of Compiled .mobi File

I’m working on a nonfiction book that has images in it. Each image is ~150 kb or less in size by itself but adds around 800 kb to the final compiled .mobi version of the book. Why does one small file add so much to the final size, and can this be reduced?

Here is an earlier thread on the topic. What has become known since then is that although you upload two books to their server, they only send the relevant copy to your readers, so what you’re seeing isn’t indicative of the book size that anyone else will be seeing after they download it.