Mp4 is literally less than 1/100 the size of a png sequence in my case of a 20 second 30 fps 3d render

Support for videos will not get added, I'm sorry.

Hi! It's not possible to render video files in Minecraft (lwjgl) without using third party APIs and I don’t want to do that atm, sorry.
Maybe I will add it someday in a future update, but no promises.