When comparing the anatomy of bison, yak and aurochs in flesh, there is one distorting factor: the fur. While the fur of the aurochs and cattle is roughly (very roughly) the same length all over its body, with exceptions on the tail and the forehead, yak have long hairs at the ventral side of their body, almost reaching the ground. Bison have a huge shock of hair on their head and neck and shoulders are covered in shaggy dense fur as well, while from the end of the shoulders onwards the body is virtually naked. The contrast is not that sharp in the wisent but still evident. These different types of coat make it difficult to compare the body shape and proportions between these four species because the life appearance of the animals is greatly altered.
It would be a funny idea to take a bison, wisent and wild yak (of the same sex of course) and trim their coat, in order to see what their body is actually shaped like. But that won’t happen. So I took pieces of papers and did some sketches. I tracked out photos of individuals of these three species showing them in a useful lateral position. Then I tried to deduce what the body beneath the coat looks like using anatomical knowledge with the aid of photos of skeletal material. The results are inserted below, and I think they might be credible. The reference photos are linked in the caption.
What is interesting is that the yak confirmed my suspicion that its body shape is superficially quite similar to that of the aurochs. Actually, the sketch could almost be mistaken for an aurochs reconstruction, and the animal itself looks like a hybrid of bison and aurochs to me.
And, just out of curiosity, I did a sketch of what an aurochs might have looked like with the coat of a bison. I took my latest aurochs reconstruction and added the masses of hair onto the head, neck and shoulders, as well as the beard.
It looks a bit funny, mostly because the head is much larger in relation to the rest of the body than in bison.