Much of the science behind The Croissant Diet and The ROS Theory Of Obesity is based on the interplay between insulin signalling and ROS production. The theory is that insulin is the signal to store fat and ROS blunts insulin signalling.
With that in mind I made this 2D graph of the different macromolecules, with my current understanding of their effects on insulin stimulation and ROS. The croissant diet is high in starch and SFA (saturated fat), so insulin will be produced but the signal will be blunted. This is, of course, the case in the the traditional French Diet and the French remained lean on it.
On the other hand, if you combine starch and vegetable oil (PUFA), you get maximum insulin product and maximum insulin signalling (very low ROS). This diet very clearly produces obesity, as is evident in the modern Middle East.