A bit of graphics/video history
The way video files are compressed (the 'co' bit of codec) obviously determines the file size. Just the same as comparing a png (lossless) with a jpeg (lossy)
The APNG is very large because each frame is a complete, stand-alone image. Your example which I downloaded as rgbTKp9.png - 92 individual frames - 12.9 MB
You could convert those to jpeg and make a video - a
motion jpeg where each frame is a complete picture 92 individual frames - 1.4 MB and if you ever want to do that using FFMPEG
ffmpeg -framerate 10 -pattern_type glob -i '*.jpg' -vcodec mjpeg out.avi
Not many cameras use MJPEG these days, so the 92 png's rendered as mp4 is much smaller 0.8 MB
ffmpeg -framerate 10 -pattern_type glob -i '*.png' -c:v libx264 -pix_fmt yuv420p out.mp4
Why much smaller, it is the encoding. The first frame is a full picture, an 'I' frame. Subsequent frames are vector references to groups of pixels, 'B' frames and 'P' frames.
Your video which I got as 7Ny06qg.mp4 was smaller again, 0.56 MB
Why? One 'I' frame, all the rest 'P' frames. Very tightly compressed, very much never to be edited again.
Back to history,
Why would anyone want to use MotionJpeg? The format goes back so far, no-one can patent it. Unlike mp4 which is owned by the evil mpeg licensing authority
see:
https://en.wikipedia.org/wiki/MPEG_LAexample: Read the small print for the video format on your expensive digital camera. Publish your video on say youtube and sometime in the future you might have to pay royalties.
Has not happened yet, but it might one day.