QImage::scaled

I am using QImageReader and QImage to load and scale extra large jpg file, which is 46240 * 45480.  When I scale the image into small images, it works well when the target size is smaller than 460 * 460. However, it will lose part image when the target size is over 480 * 480. Please refer to attached images as the result of scales.

image_reader = QImageReader(file_path)
image = image_reader.read()

overview_image = image.scaled(480, 480, Qt.KeepAspectRatio, Qt.FastTransformation)
overview_image.save(file_path + '_bk01.jpg')
self.image_model.overview_pixmap_m = QPixmap.fromImage(overview_image)