[Buge]

>there are many encoders of varying quality which can generate such files and each encoder has tons of settings. I don't know which encoder macOS uses but ffmpeg includes state-of-the-art H.264 encoder (x264). Simply reencoding video from H.264 to H.264 using ffmpeg can give you better quality/size ratio than the original file.

It's actually even more complicated than that. Each encoder has multiple settings on how much CPU to spend on the compression. Live screen recording (which this was) usually has the setting set to spend little CPU and get little compression. So like you said you don't need to change which format you use to get better compression, but you also often don't even need to change what encoder you use, you can just change the compression setting to use extra CPU (which might not be possible on a live recording).

[astrange]

Another factor is keyframe rate, which makes the bitrate requirement much higher at the same quality but is needed to let people join the stream more than every 30 seconds.