In order for the materials of an object to differentiate by density, the body must be molten or fluid at some point in its history. The energy generated by the formation of planets was large enough to melt the planetary materials so differentiation could occur. For smaller bodies in the solar system the heat generated by their formation was probably not great enough to melt the body. Also the reduced gravity of the smaller body would not attract the denser materials to the center of the object. Venus remains geologically active - volcanoes still exist on this planet.
Volcanoes renew the surface material and obliterate older surface features including craters.
So Venus, with a younger surface, should have fewer craters per unit area. Looking at Appendices 7 and 8, find the satellite whose diameter is the largest fraction of the diameter of the planet it orbits.