Off hand, I'd say that ol' Neandy could have had one whopping big colon in there. Yet another link to Hillary.
But the rib cage isn't protecting the intenstines, but mainly the lungs, heart, spleen, stomach and most of the liver.
I have read that their bone mass was denser. Maybe their lung mass was larger to compensate for all the extra weight. All in all a less efficient model. Didn't work as well as the competition and evolution is a tough task master.