I think the shape and composition of the tunica will be the most important variable. Basically, the balloon gots two layers, one expand the penis lengthwise, the other one circumferencially. When you inflate the penis, one of the layers will be less elastic and will expand less. It varies from a man to another probably. And the rate of growth could also be diferent between two layers. There are guys who don’t get girth whatever they do, so much that they gain only length with clamping.
The PE techniques used can make a difference too; we can work length without working girth, but for the other way around, we have no techniques : any time you work girth, you are also working length.