Why there are more and more big newborns
Because more and more are able to be born. I know plenty of extra large babies are birthed naturally, but it is a fact that a C-section can allow a child to be born that might have otherwise, decades ago, not made it out of the birth canal alive. And that child then has the chance to grow up and have children of its own. Which gives the being-large-at-birth set of genes better odds at being passed on to the future offspring
Also, women who give birth to a large baby now are more likely to not only to live through the experience, than decades ago, but also are more likely to not be traumatized by the experience, and there is a greater chance that they will go on and give birth again, and chances are, they may have another large baby.
I collect antique gynecological and obstetric books and manuals. Many a full term infant was diced up piecemeal and drawn out with blunted hooks to save the life of a mother. And many a mother held her living baby as she bled to death from the incisions made to free it from her body.
Bigger babies survive more often now, which is why we may being seeing more of them.
I don't feel it has anything to do with particular foods or chemicals.