I've tried several times to figure out this file size curiosity. I ripped a McDonalds spot from Facebook today and dropped it into FCP to check out the info. It's a 15-second spot that was saved as an mp4, with a resolution of 1280x720, and the file size is only 1.2MB! I took some footage I shot with my DSLR and snipped it down to just 6 seconds as an experiment, but every possible combo of outputs I try can't get this looking decent at less than maybe 5MB. And mine is only 6 seconds. How in the world was that McD spot compressed so much and still look so good? Amazing!