I think you are wrong here. Your calculations about what file size shoud be sufficient would be right if I would do my compressed encoding from the same source material that the original encoding was done from. That would mean the raw, uncompressed data from the camera’s CCD sensor.
As far as I know the streams that we get are the encoding done internally in the webcam, set to a specific bitrate. And that biterate sets the level of inaccuracy that the raw CCD data is represented with.
If I want do make my video with the best possible video quality, I need to represent that inacurate representation as accurately as possible because I am actually adding a second layer of inaccuracy on top of the first one.
Unless I’d preserve the original video encoding in my file. But there’s not much you can do with a video that way. With my tools I know how to trim the length of my stream recordings and that’s it. I think there’s programs with which you can put several equally encoded clips together into a new file while preserving the code. Not sure if you can make changes to the audio, how those two streams are actually interwoven. But if you make any other changes to the video, cropping, resizing, adding filters or logos, then you definitely need to encode anew, adding that second layer of inaccuracy.
So I did an experiment, recreating an equivalent video to my original post at 20 Mb size.
I don’t save project files of these mini projects. So this is not my original edit, I just used one continuous chunk from the same recording, same length as my first video, with the same music and resizing, as accurately as I could do by hand. should be good enough for a comparison.
Here’s the video settings I used.
To the best of my knowledge this should give the best quality I can get aiming at a certain file size: Average bitrate and dual pass encoding. In my first video I had set the video stram to 3300k and audio to 128k, not too much for music, so I had a combind bitrate of 3428k
I calculated the new video bitrate (3428 x 20/94) - 128 = 601.3 and gave it a generous 605 kbps
(Talking about file sizes, are you using a Mac, too? You said my video was 94 Mb, as indicated on my Mac, but I thought on a Windows system it should be indicated as 94.046.788 / (1024 x 1024) = 89,7 Mb?)
Anyway, the file landed at 20,3 compared to the original 94, as indicated on the Mac. And now here’s some comparison shots. The small video is on the left, my original on the right side.
Admittedly, it isn’t much of a difference, but if you look closely I think you’ll find that the bigger video is a little bit crisper. Look more at the low-contrast moving areas, not so much the high-contrast or still ones.
Is 94 Mb a bit of overkill for this little video. Maybe. Probably. But you already know that I don’t care too much about that. And it is better than 20.
It is my old habit, if I bother to adjust the bitrates of video encodings, to aim slightly above 90 and safely below the upload file size limit. For the accurate representation of already inaccurate images, as explained above. I even worked out a formula for average bitrate video encoding, a good rule of thumb:
Maximum combined (video + audio) bitrate in Mbps = 8 / video runtime in seconds
There you have it. You see, it isn’t so easy to make me change my ways.