Why are .anims so big?

I am in the process of recording dialogue via FaceCapX and an animation recorder script that checks for changes between frames and if there is one it saves the value of 52 blend shapes, as well as head transform rotation and position.

To my limited understanding, the file should be containing something like (int) frame + (double) blend shape value per frame.? Which would lead to something like:

in byte: 4 + (8 * 52 blend shapes) * 60 frames per second * 60 seconds = 1,497 MB per minute.

Anyway, I end up with 18 MB for roughly 15 seconds of recording.

What is missing in my calculation? Is there a bug in unity .anims?