It may sound complicated, but that universal relationship is the same one that describes crumpled wads of paper—as Herculano-Houzel showed by scrunching up sheets of paper of different sizes and thickness at her dining room table and measuring their surface areas. The relationship comes about because the bent-up paper settles into the configuration that minimizes its energy. So presumably, in folding, the cortex also simply settles into the configuration of least mechanical energy.