A kilobyte is 1024 bytes and a megabyte is 1024 kilobytes. When a form says "maximum 100 KB", it means the file on disk, not the pixel size. Two things control that number: how many pixels the image has, and how hard the JPEG encoder squeezes them (the quality setting).
This tool changes quality first, because it is invisible at normal viewing sizes, and pixels second, because that is what makes an image look small. It shows both numbers before you download.