Setting an scene break image to 100 is generally going to be a much larger size than recommended for these types of images, which are traditionally meant to be small section break indicators in a book (unless you've uploaded your own very high-quality image to then scale up as needed). So the remedy here will be to head back into your Atticus project and adjust that scene break size down a bit.
Then when you export a new PDF file, just be sure to double check how the break image is appearing there before ordering new copies of your book, as the way the image is appearing in your PDF will directly translate to how it's printed in a book.
To adjust the size of your ornamental breaks you'll want to open the book in Atticus and click on Formatting from the top center. You'll then want to locate the theme you are using for this book and click the three vertical dots to the right of that theme title and select Edit. You'll be able to reduce the size of the scene break under Scene Break settings.
You can learn more about creating/editing a custom theme here: https://www.atticus.io/create-a-custom-theme/