Image mirrored verticallay reduces file size

[novomontana]

I recognized a sort of miracle: When I mirror an image vertically and then save it, the file size reduces from 1.990 kB to a mere 432 kB. Original and imaged pircture show 4.31 MPixels as original files size (digital camera image).

Has someone an idea, how this miracle comression can occur?

[Bhikkhu Pesala]

There is nothing miraculous about it at all. If the original was a JPG image compressed at high quality, and the new image is compressed at a lower quality, it will get smaller. JPG compression is very efficient. A setting of 75 in the file save dialogue will give a much smaller file size, without any easily discernible loss of quality, (unless the image contains text or other sharply contrasting areas). A closer look, though, will reveal artefacts.