The lower leg bones fall into the category of function equalling form. The lower leg looks like it does because of it's function. First, the tibia is there to support the weight of the body above it and locks in nicely with the femur. The tibia changes shape from top to bottom from being a three-sided prism to a four-sided block at the ankle. The fibula bone starts lower than the tibia and ends lower too. The fibula doesn't carry any weight but is used for the origin and insertion of muscles. That's why the side of the leg with the fibula is more shapely. It has more muscles. The lower leg is the same height as the upper leg and the shape to the calf muscles are around 1/3 of the way down the lower leg.