Interlacing doesnt store another image, it simple changes the order in which the images lines are sent and rendered. The server sends every Nth line, reaches the end, then goes back to the start, reading the lines in between.
After each pass the browser displays the downloaded lines, plus filles the lines not yet received the same, but with each pass the gaps being filled get smaller and the image sharpens. After several passes every line has been read, and the browser has rendered the image in full detail.
Hope that makes sense, it does explain why there should not really be any difference in filesize, so I cant explain why some people have observed a file size difference.
As I understand it, there is only 1 bit in the file which says if its interlaced or not, and the server and client (browser) just handle it differently if it is set to 1.
