  We present a fine grid of solar metallicity models of massive stars (320 in
the range 12$\leq$M(\msun)$\leq$27.95), extending from the Main Sequence up to
the onset of the collapse, in order to quantitatively determine how their
compactness $\xi_{2.5}$ (as defined by O'Connor $\&$ Ott, 2011, ApJ 730, 70)
scales with the Carbon Oxygen core mass at the beginning of the core collapse.
We find a well defined, not monotonic (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 C 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 \nuk{C}{12} left by the He burning play a major role in sculpting
the final Mass-Radius relation, it is the abundance of \nuk{C}{12} the ultimate
responsible for the final degree of compactness of a star because it controls
the ability of the C burning shell to advance in mass before the final
collapse.
