It's not all linear; each 20% interval from 20-80 is different size increments, having no arms adds 10% and having no mouth adds 5%. If you check the non-cock bits of the art, you'll see the size difference changing a bit between increments.
Didn't saw any size difference, but still...: Normal Head and arms at 60%, no mouth and no arms at 55%, back to normal head and arms at 50%, it goes on until reaching 30% which is the lowest with Bret when starting. It just seems weird that it goes like that without the runes doing any to cause it. Could be a bug, but guess wait and see how it goes.