What is the largest part of a horse's hoof's bottom surface?

The largest part of a horse's hoof's bottom surface is indeed the sole. The sole is a concave structure that makes up the majority of the hoof's underside. It is responsible for protecting the sensitive internal structures of the hoof and providing support as the horse moves.

The sole's surface is flat and covers a substantial area, allowing it to serve its purpose effectively. It is made primarily of hard keratin, which contributes to its durability and ability to withstand the pressures of weight and movement. While other parts of the hoof, such as the frog and the wall, play important roles in hoof health and function, they do not occupy as much surface area as the sole. The frog, while significant for shock absorption and traction, is smaller and situated centrally in the hoof's underside. The hoof wall, though crucial for protection and shape, primarily encases the hoof and does not contribute to the bottom surface area like the sole does. The periople is a thin band of tissue that contributes to the hoof wall but is also not a major component of the bottom surface. Thus, the sole is the predominant structure noted for its size and role in the overall function of the hoof.