Well, I don't think there's anything you can do TBH.
Our babies were,

DD1 - 3.320kg (7lb5oz) 8 days post dates
DD2 - 3.885kg (8lb9oz) 7 days post dates
DS - 3.840kg (8lb7.5oz) 2 days early
DD3 - 3.450kg (7lb9oz) 5 days post dates

So no matter when bubs is born, they will just weigh what they weigh. I am not a big person. I do not believe for a second that your weight or gain have anything to do with it. I put on minimal amount of weight (5 - 6 kgs) in each pregnancy, but had good sized babies. They all have the same father & didn't get bigger the more I had as people expect.

Smoking 'may' make a difference, but I highly doubt you'd want to go there, & a friend of mine's DS was 10lb 10oz at 16 days post dates, even though she smoked through her pregnancy. Another friends bub was only 6lb9oz, less than her first, at 7 days post dates & I believe the smoking may have had an affect there.

Its all about the baby itself & chance. All I can say is do what you can do to have a healthy baby. Your body will take care of the rest.
I really don't think bub's size was the issue. The position of bub was more likely the problem.

What ever happens, good luck with your birth