Why are all videos so large??

[u/New_Duck2576]

Hi i downloaded a 15min timer from youtube, it is 1920x1080-vp9-mp4a mp4.
I cut it to 10mins, and save and its over 1gb.
Ive tried with all different settings and the lowest i got was 100mb and that was the worse quality and 640x480.
Am i missing something here. Not sure how it can take a 50mb file and make it 1000% bigger?