>>114188
> I know it is a container.
> [...]
> FLV seems to be 3x size of the mp4 recoded version of same file.
You don't seem to know what this "container" thing is. But that's alright. We can all learn. :)
Most commonly a video file is only made up by two things: Video and audio.
Your video and audio codecs, like H264 and AAC, reduce the file size of the raw video and audio recordings.
For instance if you make a video at 24 FPS and your image stays the same for an entire second, your video codec will be able to say "Here's an image. Video player: Go show this. And then repeat it 23 times."
That way you've just - easily - reduced the file size (at this one point) 24x.
Of course there are many more tricks that a video codec employs - The same is true for audio codecs.
But now you got a problem: You have a video and an audio file. How do you combine those to have a file that plays both?
And that's where container formats shine: They shop video and audio streams into small, easily consumable pieces and interleave them throughout the file in some way.
There's basically always a few frames of video, followed by a split second of audio and so on.
That way you can easily stream a video file from e.g. the internet, without having to load the entire file first.
It also allows you to play a 2GB video file from your disk without lagging and without having to load it into memory first.
Container formats usually do some additional stuff on top of that, like allowing one to skip around in the video, or storing subtitles.
The "Juicyjackie SuperSized Shaving.mp4" file that has been shared recently here is a MP4.
If you open this file in e.g. MediaInfo you can see that this 59.6 MiB file consists of a 52.7 MiB H264 video stream and 6.09 MiB AAC audio stream.
These make up 99% of the file size. Only the remaining 1% is being used for this "container" thing called MP4.
Now the same with "Update 321-Work it Out.flv":
The file is 144 MiB in size and made up with 138 MiB in video (encoded as H263) and 1.81 MiB in audio streams (encoded as MP3).
These make up 97% of the file size . The remaining 3% fall to the FLV container.
This means that a file encoded as FLV is only roughly 1.6% larger than its MP4 counterpart.
The reason for this is that FLV is a much better fit for streaming, but that doesn't come cost free. For instance you can't easily play a MP4 from the middle of the video without having loaded everything before this point.
The only reason this works on Youtube is because they conjur a newly generated MP4 for you every time you skip around in a video. (And in modern browsers they don't use MP4s at all anymore.)
The modern alternative to FLV is MKV, but there's only limited support among browser players for it.
Here's a repost bonus for reading:
https://we.tl/t-uInLOQXKCH