This is an interesting question and I have to chime in because I think that I know where the saying came from. Each successive baby that a woman has tends to be larger than the last(gross blanket statement) on average. I know that there are different circumstances that can change this but it seems to be the general rule, and one that I have seen proven true in many instances.
My oldest friend has two children and her daughter, the younger one, was 9 13 and her eldest, the boy, was 7 7. My younger sister also has two children, a boy and then a girl, which also followed the 'general' rule.
Then there is the adults that I know, in my family there are three girls born to both my mother and father and even though my younger sister was the largest birthweight, my older sister was the middle birthweight and I was the smallest. I was also a month premature so that does make a difference. But now my older sister is the tallest and my younger sister is the shortest. But then the trend is reversed in my husband's family, the youngest of four brothers is the tallest and then they get incrementally shorter until you get to the eldest who is the shortest. I beleive that their birthweights followed the same pattern.
So I guess that you could say that humanity is still wonderfully varied <g>
