JPEG size made smaller by MSPAINT, why?

Even as you can't tweak a quality value when you save a jpg file in MS PAINT, it applies a default value that it's unknown (around 75% - 85%). This value will degrade the quality and reduce the size of the file.

The image file gets smaller and loses quality. If you view the image and zoom to more than 100% you can usually see the artifacts caused by the JPEG compression as in this example (a leaf in foreground and sky in the background):