Why is the medial cuneiform a different shape to the other 2 cuneiforms

How does this contribute to the transverse arch of the foot?

The shape of the body is geneticaly determined (because of the evolution) including the bones of the foot. So the articular surfaces of the medial cuneiform is also genetically determined and joins to the neibouring bones strengtened by ligaments and muscles tendons give rise to an arch. This arch is one of the more criteria of the so called flexible support, which is a condition of the dipod movement.