The sole explanation is that TA is stretched. Being stretched axially gives more room for circumfenrential expansion. No in vitro studies has shown that smooth muscle hypertrophy leads to bigger penis size, whereas surgery using tunica grafting without increasing smooth muscle content leads to big gains in size.
“The smooth muscle of the tunica”? You don’t have even clear what you are speaking of.
I did not get much sleep last night, I had to work late. I meant the smooth muscle *inside* the tunica.
How can axial stretching cause circumferential expansion? Imagine attaching a lead weight to the end of a rubber tube. Would that lead to circumferential expansion? Obviously not.