Why 20 KB comes up
20 KB is a tight ceiling for a tiny avatar or a form that only accepts a very small photo.
What happens to quality
Most camera photos will lose detail and usually need smaller dimensions as well. Check the preview. A lossless PNG will often stay larger than 20 KB.
Why some images cannot reach 20 KB without resizing
If the search would have to make the image unusable, it stops at a documented minimum. The result is at or below 20 KB only when that limit allows it.
WebP is the default output on this page because it is often smaller than JPG and much smaller than a lossless PNG. You can switch the format if the destination requires JPG.