I'd guess animals have a lot of compute dedicated to finding out the shape based off of various visual cues: Lighting changes indicating edges Gradients indic

I'd guess animals have a lot of compute dedicated to finding out the shape based off of various visual cues: Lighting changes indicating edges Gradients indicating a smooth curve Texture maps that discern between matte, smooth, translucent, often in depth enough to determine the texture material itself. Reflectivity Changes in reflectivity when reorienting oneself or the object Binocular Vision Parallax effect between frames A large database matching objects to memories and experiences Many other techniques for tracking and seeing movement. In the above, Parallax and Binocular vision, and consulting one's memory are the only ways to determine the sizes of the objects being looked at. It's why those optical illusions tiny/big rooms work so easily; neither we nor CV algorithms would be able to discern the sizes of objects without prior memory of such objects or by walking around it.