You can make webms and mp4s and gifs to the exact file size you want with something like the movavi video editor. You normally have to pay for it but heres a link to the cracked torrent version with lots of seeds.

https://1337x.to/torrent/1756771/Movavi-Video-Suite-15-4-Crack-Portable-Crackingpatching/

There's a lot more that you can do with the movavi video suite than making webms or gifs too.

League recordings should have a replay of the persons view directly, not just a replay of the game itself. It's a lot more important to see mouse and menu movement than just gameplay alone. The way I used to record league gameplay is through OBS