How the photo gets under 15 KB
A file this small has room for far fewer pixels than a phone photo has. The tool first lowers the JPG quality; when even its lowest quality is over 15 KB, it makes the picture smaller step by step until it fits, and says so under the preview. Check that a signature's strokes or a face are still clear before you upload.
The download is a new file made in your browser from your photo. Your photo is never uploaded, and location and camera details (EXIF) are left out of the download.
KB or KiB
Here 15 KB means 15,000 bytes. Some forms count a kilobyte as 1,024 bytes; for them choose KiB in the unit menu, and the limit becomes 15,360 bytes, 360 bytes more. If the form does not say, keep KB: a file under 15,000 bytes passes either way.
If the form also sets a minimum or a pixel size
Open Form requirements and format and enter the smallest size the form accepts, its width and height in pixels, or both. A photo that is already smaller than the minimum needs the opposite job: increase image size in KB saves it again with less compression. Meeting the size does not mean a form will accept the photo, so check its other rules too.