Example: Take the PNG, convert it to another lossless format. Then convert that lossless format back to PNG.
Different file size, right? OK, the comment accounts for about 120 bytes.. but what about the rest? Why is it that much bigger?
"As I read, numbers I see. 'Twould be a great shame not to count this art among the great texts of our time" - count this art among the great texts... it's a one time pad, isn't it.