Huh, interesting, I hadn't realized. This link here was a very quick, straightforward description of the actual proportions: https://bootcamp.uxdesign.cc/drawing-a-flat-piano-keyboard-i...
"Update 13 October 2021: This method is great for getting piano key proportions very near accurate with a few very simple steps. Thanks to Rob Brown who added more details in the comments that will be helpful to anyone looking to make a highly accurate model (e.g. if you’re trying to align a drawing with a photo)"
The actual proportions are described in the link above (I wrote a little algorithm to do it accurately for my music app)