Why do two of the same downloaded picture in windows photo gallery coming up with different sizes in propertie?

I always seem to end up with two or three of the same picture downloaded in windows photo gallery from downloading a chip two or three times. I notice when I do this the first will have one number and a certain kb size and the next and any thereafter will have a different picture number and a smaller kb size for the same picture. Why would this be?

JPG’s, if they are re-saved, will become more and more compressed. This results in smaller file sizes and decreasing quality of the image. You may hear of a jpg artifact, this is the resulting distortion of colors and edges as the data is lost as jpgs are re-compressed.