Thanks for your inquiry. In your case, I suggest you to use HtmlSaveOptions.ScaleImageToShapeSize
to save your document to epub file format. This property specifies whether images are scaled by Aspose.Words to the bounding shape size when exporting to HTML, MHTML or EPUB. Default value is true.
An image in a Microsoft Word document is a shape. The shape has a size and the image has its own size. The sizes are not directly linked. For example, the image can be 1024x786 pixels, but shape that displays this image can be 400x300 points.
When ScaleImageToShapeSize is true, the image is scaled by Aspose.Words using high quality scaling during export to HTML. When ScaleImageToShapeSize is false, the image is output with its original size and the browser has to scale it.
If you still face problem, please share your document with us for investigation purposes.