of course it is our head. to give us an idea of the position of a sound on the left, one big parameter is how long it takes for that sound to reach the right hear after it reached the left. if my head is bigger, that lag will be bigger in everyday's life sounds. so I will need it to still be bigger if I want to feel like it's real when using a crossfeed.
another parameter is how much attenuation there will be between the left and right at different frequencies, that depends on the angle it's supposed to be coming from, the shape of the head, the size of the ears, how much will bounce from my shrimp like torso.
we all have different experiences in real life because of different morphologies. so when we end up with the same music into the same headphone, we don't necessarily have the same experience because we don't necessarily have the same reference of what reality sounds like. that's what I was going at in last post.
of course different headphones just add to the mess when in search for a close to realistic experience.