My brother and I share the same dad and mom. My brother has always taken after my dad's side of the family in almost every physical aspect, including penis size. Whereas I have always taken after my mom's side of the family physically, including height/weight/penis size. I've seen my grandpa and my uncle naked over the years, our penises are definitely growers. I'm short and stocky, my brother is tall and lean.
I dont have children so cant really testifiey to that but i am smaller than my father but we are both on the smaller side. I am 3 inches he is about 5. My mothers side of the family were big guys my uncle i know was 7.5 and he said my grandfather was a 9. My nephews fathers are big cocked guys and i know my eldest nephew is defo taking after his father.