Compress to 20 KB only fits a small graphic or a heavily reduced photo. The same search runs: quality first, then smaller dimensions, and it stops before the picture becomes a few pixels. If nothing lands under 20 KB, you still get the smallest file and an honest note.
The quality slider re-encodes a JPG or WEBP at the percent you pick, and writes a PNG again when the file is a PNG. PNG has no JPEG-style quality step in the browser, so that slider does not change a PNG. Target-size mode binary-searches quality, then shrinks the long edge if even the lowest quality is still too big. A PNG that cannot land under the target is saved as a JPEG, and the note says so. Strip EXIF is on by default. Re-encoding already drops metadata; unchecking the box copies a JPEG EXIF segment back onto a new JPEG when one was present. It cannot put JPEG EXIF into a PNG.
Use 20 KB when a form demands it. For anything you still want to look at, 100 KB is the gentler page.
Before and after sizes are the real byte counts, and the previews are the pictures the browser produced. A batch downloads as a ZIP. This is not compress PDF. A PDF redraws pages and makes text unselectable. A photo should stay on this page. The file converter changes format without chasing a kilobyte target. A target is a best effort: if the picture cannot get that small without becoming a speck, you get the smallest attempt and a sentence that says the target was missed. The page does not label a 400 KB file as 100 KB.