title_body: Which size is taken into account by Amazon when calculating the download fees
upvoted_answer: 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 file?
  
  From the first article: 
  
  
    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 &amp; Pricing page at your KDP Bookshelf.