I don't think there is any relationship between them either.

DS - 39+1 - 8 pound 2
DD1 - 42 weeks - 8 pound 11
DD2 - 41+5 - 8 pound 10
DD3 - 41+4 - 7 pound 12.

If DS was left in there longer, he would have been around the same as DD1 and DD2 I imagine. I think DD3 was lighter because of my gall bladder issues during her pregnancy and I couldn't eat like I did with all the others. All the others I put on around 22kg - in hers I lost 7kg.