It is not just about the size. There are risks going too far over your due date. There is a very slight increase in maternal & fetal morbidity and mortality 7 days after your due date. This goes up significantly when you're two weeks or more. One of the reasons is that the placenta changes or ages. In some cases, the fetus actually gets smaller because of decreased nutrition. Because of this, many OBs recommend inducing 1 week past the due date, and they are very uncomfortable if the baby goes two weeks past.

In the past (my mom's era), OBs often let go past their due date because few knew exactly the baby's "due date." With increased technology and ultrasound, physicians have a better understanding.

If this is something that you're concerned with, please talk to your OB about it. I did and it helped a lot.