I have seen my Dad's cock, and it always looked big to me. But then, I was a young boy and anyone else's cock looked bigger than mine. I think you have to consider that perspective. I have seen my two older brothers naked. I am the same size as one and bigger than the other. I think logically, there are manny factors involved in Penis size. hereditary of course and not just from mother or father. It may go back further. i think it is reasonable, (and statistically it is true) that a man 6 1/2' or more tall with large feet and hands large enough to hold a basketball one handed and in good physical health and condition will have a longer penis than the norm. However, Physical conditions such as overweight, Diabetes, heart disease, can cause low blood flow to the penis and prostate resulting in 'weak' erections of small size. regardless how big the man is.
Harry