AbstractWe present a fine grid of solar metallicity models of massive stars (320 in the range 12 ≤ M(M⊙?>
) ≤ 27.95), extending from the main sequence up to the onset of the collapse, in order to quantitatively determine how their compactness ξ2.5, defined by O’Connor & Ott, scales with the carbon–oxygen core mass at the beginning of core collapse. We find a well defined, nonmonotonic (but not scattered) trend of the compactness with the carbon–oxygen core mass that is strictly (and mainly) correlated to the behavior, i.e., birth, growth, and disappearance, of the various carbon convective episodes that follow one another during the advanced evolutionary phases. Though both the mass size of the carbon–oxygen core and the amount of 12C left by the central He burning play a major role in sculpting the final mass–radius relation, it is the abundance of 12C that is ultimately responsible for the final degree of compactness of a star, because it controls the ability of the carbon-burning shell to advance in mass before the final collapse.