Can you explain in more detail, how you are trying to use the downloaded photos?
When you download the unmodified originals on your Mac, are you saving them in a folder on your desktop?
Have you checked the resolution of the downloaded photos with the Preview.app? hen you open a photo in preview, you can use the Tools menu: Tools > Inspector to see details about the downloaded file.
- What is the format of the files? Are they RAW files or JPEGs, or HEIC files?
- What are the width and height in pixels, when you download them?
- Have the images in question been taken with the iPhone? If yes, how have you taken them? As a standard photo?As a Live Photos? Portrait Mode?
Any details you can give us might help to solve the puzzle.