Those white outlines are most likely sharpening halos. They happen when the image has been sharpened too aggressively, either in-camera or during editing. Sharpening increases edge contrast, and if it is pushed too far it creates bright outlines along borders.

You notice it more when zoomed in because the halo is easier to see at pixel level; when the image is smaller, it becomes much less visible.

To reduce or prevent it:

• Lower the sharpening amount in your camera settings if possible.
• Apply less sharpening in post-processing.
• Use more selective sharpening, so strong edges are not over-accentuated.
• If the original file already has halos baked in, they can be hard to remove completely.

Many smartphones and cameras apply sharpening automatically, so checking those settings is a good first step.