The penis contains significant collagen — particularly in the tunica albuginea (the fibrous sheath surrounding the corpus cavernosum) and in the connective tissue throughout the shaft. This collagen is produced by fibroblasts that express receptors for collagen-stimulating peptides including GHK-Cu.

If collagen-stimulating peptides produced meaningful additional collagen deposition in penile tissue over time, this would theoretically increase girth — because more tissue mass = more circumference. The glans specifically has a high collagen content and might respond to collagen stimulation in a way that produces visible thickening.

The problem: we don't know if the collagen response in penile tissue is meaningful, how much collagen deposition is needed to produce measurable girth change, or what the time course looks like. This is biologically plausible but unproven.

— BioResearchBen