Most recent studies of crater morphometry on Mars have addressed large craters ( D 〉5 km) using elevation models derived from laser altimetry. In the present work, we examine a global population of small (25 m ≤ D ≤ 5 km), relatively well-preserved simple impact craters using HiRISE stereo-derived elevation models. We find that scaling laws from prior studies of large simple craters generally overestimate the depth and volume at small diameters. We show that crater rim curvature exhibits a strong diameter dependence that is well-described by scaling laws for D 〈1 km. Above this diameter, upper rim slopes begin to exceed typical repose angles and crater rims sharpen significantly. This transition is likely the result of gravity-driven collapse of the upper cavity walls during crater formation or short-term modification. In addition, we identify a tendency for small craters ( D 〈500 m) to be more conical than large craters, and we show that the average cavity cross-section is well-described by a power law with exponent ~1.75 (neither conical nor paraboloidal). We also conduct a statistical comparison of crater subpopulations to illuminate trends with increasing modification and target strength. These results have important implications for describing the “initial condition” of simple crater shape as a function of diameter and geological setting, and for understanding how impact craters are modified on the martian surface over time.