From The Tour de France Official Guide 2012 p. 142:

 

 

“No handlebar in the world has such a diameter. The diameter is the main factor of magnitude in the moment of inertia of the section of the handlebar. The moment of inertia is what defines the rigidity of your handlebar – what all riders are looking for, as insufficient stiffness causes dissipation of your precious energy. The larger diameter also allows for better placement and lower specific pressre, resulting in reduced hand fatigue. M35 and TRENTACINQUE for a completely new feeling while riding.”