Hi António,
I would be willing to bet that all those videos are recorded in a much higher resolution than what is needed, especially for a file that is just going to be stored for five years.
Before I retired I used to do lots of videos in my advanced computer applications class. We routinely converted them from the camera's digital video format to a much small HTML5 compatible format we could use in Moodle.
Since it has been a while, out of curiosity I just converted a digital video of 1 hour and 4 minutes duration. In digital format it was: size 12.9 GB. Frame width 720, frame height 480, data rate 28854kbps, total bitrate 29878kbps, frame rate 29.97 frames/second, audio bit rate 1024kbps, 2 channel stereo, and audio sample rate 32.000 kHz.
I converted with the free version of Any Video Converter set to convert to HTML5 MP4 Movie. I wound up with a video of one fourth screen size at 320x240 and a frame rate of 25 fps. The biggest gain and reason for doing the conversion - the file size decreased from 12.9 GB to 0.422 GB or 422,661 KB.
That's just over 30 times smaller! And, a much more reasonable size to archive for 5 years.
One nice thing, the conversion on my 7 year old desktop took less than 5 minutes. Another nice thing, Any Video Converter can be set up to batch convert. I remember doing up to 16 at a time as my class size was 16 students.
I generally used videos of this size and quality as 320x240 was perfect for most of the tablets and phones that my students used to access the files from out of the classroom, to show to their friends and parents. It also meant that they were small enough to give each student the whole years collection of videos on a thumb drive to keep as a memento.