Sorry nads, the penis shaft and scrotum are not made of only skin, but underlying smooth muscle (Dartos muscle) and fascia (Colles fascia). Circumcision takes a big piece out of the middle. Refer to http://foreskin.org for a better understanding, then refer to the galleries of some of the guys on this site who are cut tight. Quercusone comes to mind.

Inside the scrotum, tiny muscles called cremasters regulate the testicles individually. Perhaps these are what you refer to as "tendons."

They cut out the middle half or so of the elastic skin system of the penis and underlying smooth muscle fascia and connective tissue. While there is a lot of individual difference, I reiterate. It has to come from somewhere.