Why 100 KB comes up
100 KB is a common ceiling for contact forms, chat apps, and email attachments that reject larger photos.
What happens to quality
A modest photo can stay recognizable at 100 KB. A large, detailed picture usually needs smaller dimensions as well as lower quality. Check the preview before you send it.
Why some images cannot reach 100 KB without resizing
If the search would have to make the image very soft, it reduces the pixel size and tries again, then stops at a documented minimum. The result is at or below 100 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.