JPEG XL is unique in that the smallest possible file is not 1 by 1 but 8 by 8 due to the header encoding [1].
[1] https://twitter.com/jonsneyers/status/1381894108067868672
[1] https://twitter.com/jonsneyers/status/1381894108067868672
> a single pixel jxl image is 19 to 20 bytes depending on the color