Although irrelevant I am also quite intrigued.
Irrelevant nothing. File extension makes a difference in the size of the file like I just said. Some are much larger than others because they preserve more data. A slight downgrade in quality is the trade-off for manageable disc space. It's the same reason that people who construct websites tend to use .jpeg and .gif file extensions for images instead of .tif and .psd extensions.
I hate to sound like a broken record, but I'm a filmmaker. It's my job to know this ****.
The fact remains that if you're trying to upload a video for showing off at Theory 11 and 3 and a half minutes translate to 80 megs, you're using the wrong file format. Upload it to YouTube and provide a link? Why create extra steps? Just save it in MPEG. Problem solved in one step.
I sometimes wonder why I bother trying to help when no one can be bothered to listen and just continue to perpetuate the myth that YouTube is the solution to everything.