I believe it is heriditary. my father was long and thick, my older brother is long and thin. he once saw my cock years ago, at age 20, and commented, "you got dad's dick that's for sure."
my oldest boy is hung. in fact, he thanked me one day for passing down my size to him. he said his wife thanks me too!! Ha Ha! a couple years prior to that, he was bragging to his mom one day that he was fairly large down there from what the ladies told him. He was surprised when his mother told him, "thank your father and his side of the family." he said he thought her side was the cock contributor, and didn't think I was that big. his mother gave him the correct info.
my middle boy is not very big, and is very shy about sex, but he is still young and developing at a slow pace. a late bloomer
my youngest is most like me, skinny with a fat dick. his one gf called him "freak" affectionately. not knowing it at the time, but she was referring to his thick dick on his thin frame, aka a freak of nature. the youngest asked his oldest brother about where did the size come from. the oldest told him, dad. my youngest is also pretty sneaky. he was banging some nice puss at 16. now he's 20.