A jpg image of any one photo is not a given size. As Jon said above, it's a lossy compression. But it is also a variable compression. When you save an image, you can save it at one of 12 different level. Low numbers have small file sizes, but you lose a lot of image data. High numbers give large file sizes, but you retain most of the image data. Most consumer level programs don't give you an option in jpg level - the have one default level.

Joe - if you are copying, use a Windows Explorer window to copy rather than thru the picture view. You can go to the menu in Explorer and choose View | Thumbnails in order to see what each image is (if it doesn't default to that).