Nice that your dealer knows everything happening in Canon HQ and is free to tell people about that. Regardless, the 24-105/2.8 came first, so suggesting that it used the barrel design of a lens coming out almost a year later is pretty silly. The reality is that Canon almost certainly developed them together and intended them to be the same size, as they did with the VCM lenses. The point is that for cinema rigs having a set of same-sized lenses is important. That's why the CN-E prime lenses are all exactly the same diameter, and 4 of the 6 are the same length, with shortest and longest focal lengths being pretty similar.