I'm here a bit late but have the same problem and was referred here by GMF.
Brief description: I have resizing turned OFF for now, and did my own resizing, long side = 2500 pixels before upload to Photos folder.
As has been described above, viewing a photo in individual view, image is pretty severely degraded. Clicking the Download button, the download image is exactly as it was uploaded, all the detail is there. I did
In view of the 720x720 discussion, I took my original image and downsampled it to long side=720. I then took small screenshots of the same part of the image; these were from (a) the Download screen, (b) the individual photo view screen and (c) the 720x720 pixel downsample on my local computer.
Comparing (b) and (c), the degraded individual image view has indeed been downsampled to 720x720, shown both by the overall similarly degraded appearance of each, but also counting and comparing the pixellated steps in each, which is pretty clear when looking at corresponding parts of the spectacle frames. Exactly the same number of pixel steps in each. Therefore, pretty certainly 720x720 or close to it.
I show the clips below:
(a) the Download screen at full resolution
(b) the individual photo view screen
(c) the 720x720 pixel downsample on my local computer
Well, that's what I have, what to do now?