Why does my dad have a bigger penis than me, shouldn't I have his genes?

Hey I'm 17 years old and the other day I walked in on my dad masturbating. He was wearing headphones and facing away so he didn't notice me, but I couldn't help but see that his penis was huge, it looked like he was fighting a pringles can. Mine is only like 4 inches and thinner than a TP roll. Am I just still growing, if my dad is that size then doesn't that make it pretty likely that mine will grow that big too?