I am technologically illiterate and I need some advice when it comes to converting and compressing files.

I've made plenty of videos that I want to share on 4chan and there're various websites where I can convert to webms but the file sizes are always huge and I have no idea how to get the file size down without losing any video quality. (I get that some quality has to be lost but I could make these videos 50x50 pixels and 1 FPS and it would still be 5MB)

So if anyone could point out some tips or even some software, I would be very grateful.