My prevailing theory is it creates a hypotenuse that makes their hypotenuse-leg look longer and your brain generalizes tgat impression/illusion to the other obliquely (and therefore: skinny-er)?
You're on the right idea, but it's got nothing to do with triangles, and more to do with lens tricks and musculature.
The weight-bearing leg is positioned furthest from the camera, allowing lens foreshortening to make the active muscles seem smaller to match the front leg. The front leg is extended without bearing weight to smooth out the muscle fibers, and in this case creates visually-interesting leading lines. Sometimes they're crossed in front too. Same effect, back leg bears the weight.