Howdy sixgunners,
I'm no gunsmith, but I believe that you all have the right opinion that cylinder diameter has nothing to do with the creation or prevention of a "drag line". I will, however, posit the primary reason for the increased cylinder diameter. If the cylinder diameter is a bit small for larger cartridge chambers, the thickness of the metal under the bolt notch can become too thin for full factory loads. The larger diameter allows for full depth bolt notches with sufficient metal to safely handle the pressure.
Now if the diameter of the cylinder is not increased sufficiently the only way to assure that there is enough "meat" under the notch is to cut the notch shallower. This might lead some to say that would cause the bolt to not enter the shallow notch reliably. Hence slipping passed the notch and allowing the cylinder to over rotate with the bolt fully up thus creating a "drag line". I suppose that someone might say that was the reason for the increased diameter, but that is hardly likely.
Them's my thoughts,
Rev. Chase