Assuming everyone's penis size is easily determined (folks are naked, for example), a man with a larger penis will mate more than a man with a smaller penis. The man with the larger penis will have more females who are interested in him (because females prefer his larger member) while the man with the smaller penis will have fewer females who are interested in him (because they mostly prefer the other guy's larger member).
Are you speaking from experience? How many kids do you have?
Wait....
I don't have any kids