Then the AVI can be run through a Quicktime converter. That process is very fast. The file saves at the same size it displays at. JPEG, on the other hand, is a compressed format. It may display at 10K, but it saves and transfers at only, say, 6K. Pretty slick.
Videos work the same way.
AVI does use a compression rate, but not as aggressive a rate as a Quicktime video. However, the AVI is a far better quality image. I don't know. That's up to you. Relative Size page.