Why the file size of the video is so huuuuuge?
This first videoclip of lesson_01 takes 752 MBytes and is 17 Minutes. That's by far too much and is no need for doing so. Most of other videos are like that.
I'm sure that reducing the bitrate to usual size won't have too big impact on the video quality. This video could be seen with nearly no difference when compacted to a size of 20-50 MBytes or much less.
Let's do some raw, approximated calc.
This introductory clip is 17:40 length or 17*60+40 = 1060 Seconds.
The size is 752 MBytes. Means each seconds go 752/1060=~5 ,6 MBytes per Second or
5,6*8 =~ 45 MBits/s.
This is the payload only. Multiply with 1.5-2 for the overall bitrate.
According to https://en.wikipedia.org/wiki/Bit_rate for HD Videos a rate of 4.5 Mbit/s is good.
Who the heck needs such rate? Ok, there maybe other people which needs 4k or 8k videos. This also means that customers with a 2Mb/s wire has to wait half eternity until stream buffers are filled up. (Yes, there are still a lot of people with such limited access).
I suggest to reduce the bitrate of all your videos to a reasonable value suited for the average. The average watches at 4k or 8k? Then offer alternative files download with a reduced bitrate. Download is faster and not so expensive.
... and of course you'll save a lot of REAL plants ;-)
I pay 10 Euros for 2GB download with my mobile rate. So this video costs me (752MB / 2048MB) * 10 Euros ~ 3,72 Euros. Only for the Lesson 01. That's too expensive. And of course the picture jerks around and download takes much more time.
Edit: Shame on me. The operation above is wrong. It should be: 752/1060=~0,56 MBytes/sec. Sorry.