The complex brain morphology emerges from intricate growth and folding processes during development. Sulcal depth is a shape descriptor particularly relevant to characterize brain development and variations, but the potential influence of global brain size is not controlled in existing estimation techniques. In this work, we address this issue by introducing a theoretical framework allowing to explicitly control for the influence of isometric scaling on sulcal depth measured at each brain location. We provide both formal and experimental demonstrations of the advantages compared to the most popular method from the literature.