Images being resized even with 'Full' quality selected?

Wondering if anyone else is experiencing this behavior - I have image upload size set to full in General View settings;

When uploading a large image (5120 x 2880px) it appears to initially be available at full resolution (when clicking the image in the detail view it opens full screen at high resolution)

But once the app has completed its sync, the thumbnail in the detail view disappears for a few seconds, before returning. When clicking the image it appears significantly scaled down, and where this image is used in the app it is at a much lower resolution. Clearly the image is being resized upon sync, not kept at full quality.

Is there a reason to this, or is it a bug?

Thanks