The backside "bulge" near the crank port is part of the solution to the dimensional problem that Campagnolo faced when the CPSC modifications were solved, the CPSC compliant cage is wider due to the forward lip, to make room, more distance was made between the large ring and the backside of the arm, pushing the rings inboard, that is why to take up some dimension, the later cranks appear "flatter" on the back side, and the spindle changed too. This is why one cannot in most cases use a later crank on an early spindle, the chainrings will often hit the chainstay and or the chainline will be off. Another reason later rings have a perimeter pin that is too long to use on early cranks unless the pin is filed shorter.