Question, do you have caching disabled when devtools is open? If so that'll cause it to continually re-download. Looking at the markup it shouldn't redownload but devtools will make that happen quite easily.
Completely agreed about the size though; geeze.
I simply asked the poster if they had caching disabled when they had devtools opened as I ran into this exact issue before. Why does the default matter if I asked them if they had it enabled?
I don't really understand what you're trying to get at with your series of replies.
As for downloading multiple times: I don't think it's happening though it looks like it.
The server is sending 206 Partial Content for the video. This is why the file downloads multiple ranges if the page is left open for a while. The first response if for a chunk of the file. The next few are for different chunks. Then it repeats.
~8 Mins on the page and I have 5 downloads of the hello-world_2.mp4 that went to the server. I have 4 that were able to be served from the disk cache.
It appears the screencast video is to blame, it is not only huge (~70MB), but it keeps getting downloaded instead of just replaying. I just checked the console again and the page is now up over 300MB downloaded. Glad I wasn't on mobile data!
Update: I downloaded the video and ran it through ffmpeg to see how much space could be saved... Original size 72.2MB, new size 1.7MB. Screencasts obviously compress very well, but this was pretty surprising. You could easily optimize this down further and probably half the size yet again.