Diana,
the opened file size is different than the saved file size because a jpeg is a compressed file format. For example, a 4X6 image saved as a .tif file at 300 dpi is 6.18 MB. The same file saved as a jpeg is 509 KB in saved file format (not opened) #8 quality setting in PS. The good news about jpegs is that they take up less space as saved files; the bad news is that if you open and close them a lot, they lose image detail because the algorithm for compressing the file loses information each time it's opened and closed.

For that reason, most people save their images for long-term storage as tiffs which use loss-less compression algorithms i.e. you can open and close the file as much as you like and you don't lose any data.

When we send our advertising files to publications, we send them as jpegs for transmission efficiency but ask the publication to open the file and re-save it as a CMYK tiff. That way, the archive copy will always be intact.

Hope that helps.

Jim