The largest element of almost any eBook will be the image content. Inside your file, the Kindle book format supports JPEG and GIF images. Images in other formats KDP accepts (BMP, PNG) or images that exceed this file size requirement are automatically recompressed as JPEG files during conversion. For the best results, you'll need high-definition images with a file under 650MB.

If you inserted images into your eBook in BMP or PNG format, you can decrease the size of your file by saving your images in JPEG format and then reinserting them. Here's how:

1. Open the file in your image editing application (e.g. Adobe Photoshop®)
2. Select "Save As" 
3. Choose JPEG or JPG as the file type

Adding video and audio files can dramatically increase file size. KDP currently supports files of 650 megabytes or less and 8,000 pages or fewer. If you try to upload a file larger than this, it will not convert.

If your file is larger than 650 megabytes, consider reducing the amount of audio and video content. You can also publishing the book as multiple volumes with less content in each individual file.