I've always heard its usually a pound or something heavier than the mother at birth (which that could freak you out - so for your sake I hope its not true!!)

But for me - I was 7 pounds, 7 oz at birth, DH was 7 pounds 14 oz, DS was 8 pound 2.5 oz, and DD was 8 pound 11.5 oz. So its pretty accurate in my case.