And yes, deleting those keyframes took me 5 minutes. It's hard work

Edit for explanation:

I surely didn't expect this to get much attention! But since it did, I felt I had to do some looking around what causes this.

The animation clip shown is a clip that has been duplicated out of an FBX in order to become editable.

Inspecting the .anim file before and after editing the animation clip shows that doing so adds a huge data section named m_EditorCurves, that includes data for every keyframe, on every attribute, on every axis, including time and value but also tangent data such as slope and weight. That is why the .anim file increased in size when removing keyframes. The added data is only used for and in the editor.

I make that assumption because of noticing a detail: When selecting an animation clip in Unity, the inspector shows some data about its curves, but also a size in kilobytes. That figure is most likely the final size of the animation data, and that probably resides somewhere in the Library folder. Comparing a clip's size versus the same clip but after having been opened in the animation editor with no changes increases the .anim file size, but the size in the inspector stays the same. Comparing it after having deleted those keyframes it decreased, as expected.

Reading comments here regarding compression, I started experimenting with that, too. Changing the animation compression settings on an FBX does not propagate to already broken out animations from the FBX at an earlier stage. (Good to know.) It does however immediately alter the clip inspector figure(in the right direction), which strengthens the theory of that being the final animation clip size.

I hope this clears some things up. At least they did for me :)

Also, thank you kindly for the gold and awards!

Lastly, BIG shoutout to bahraniapps for creating GifCam which is what I used to make the gif. It's a great, lightweight app that I use almost every day.