So, after doing more research it's still unclear why the increase in girth (and length too?) happens. I research some cases of sickle cell patients and cases of megalophallus.
I think the growth is more as a consequence of frequent priapisms. Not necessarily smooth muscle growth. It's been long thought that megalophallus is a result of fibrosis from the corpus cavernosa.
However, there have been cases of megalophallus ocurring without the loss of erectile function or formation of cavernosal fibrosis.
Therefore a theory has been proposed in that it was actually hypoxia that caused loss of elasticity of the tunica:
"We propose an alternative mechanism for this
sequela. The increase in penile circumference follows a sudden and permanent loss of elasticity of
the tunica albuginea, brought about by a particularly intense priapism and a very engorged organ.
The loss of elasticity of the tunica albuginea releases constraints on the corpora cavernosa, which
then expand like a sponge."
https://sci-hub.se/https://doi.org/10.1016/s0022-5347(17)58582-1https://sci-hub.se/https://pubmed.ncbi.nlm.nih.gov/10962334/Thus i think this is why some people experience growth, while others don't. It is dependent on the number and level of priapisms of the patient.
Thus I'm guessing that smooth muscle cells simply have a larger expansion limit by default and are simply usually limited by the tunica. A few things i'm trying to understand are how this is related to penile shrinkage in patients with chronic ED who do not get erections at all.