jpeg artifact

JPEG uses lossy compression technique which reduces the size of multimedia files making them easily transmitted content. This implies that some original information is lost, possibly affecting the image quality. Also, each time you save your JPEG images, the quality reduces subsequently. They are the unwanted side-effects of processing an image.You can use JPEG Repair software if you want to remove any unwanted corruption or distortion in files and have better quality images

Maybe this article will help you understand what jpeg artifacts are and why removing them creates a smoothing out of the image resulting in a lower file size.