A file's size can't be changed by renaming it. To turn a 5 MB photo into a 200 KB photo you have to re-encode it with fewer bytes, and the only way to do that well is to lower the JPEG quality a little and, if necessary, the pixel dimensions. Done properly the photo looks the same on a phone screen; done badly it looks blocky.
The converter here picks the highest quality that fits your number and only reduces pixels as a last step, so the result is as close to the original as the limit allows.