You might find this article https://www.52novels.com/from-the-ebook-formatting-files-why-is-my-mobi-file-so-huge/
and this post to be helpful.
How can I reduce the resolution of images in an epub or mobi
Amazon understands this is unfair. After all, it prefers
high-quality ebooks that play nicely with the various Kindle
ereading systems.Rather than penalize authors and publishers for
making beautiful full-featured ebooks, it calculates the delivery
fee based on the smallest version which is usually the KF7 version.
You can verify this by checking the Rights & Pricing page at your