Possibly it could not use 100% of the CPU so the phone does not go up in flames and it runs efficiently.
Most videos are large because the compression is poor. I was able to do some extreme compression of the video with far smaller size and it still looked great using Handbrake. But good compression takes time and far more calculation than fast compression.
Recording in higher res and then compressing works far better than recording in low res. I can even get smaller 4k compressed AV1 videos than the HEVC 1080p videos if I select a lower res.
All transcoding programs are FFMPEG with a GUI on top in essence. I mostly wondered whether there was an out of the box solution for Android. Don’t feel like hacking it together myself.