Hey Snowi
Sadly, I am not a size 6 myself but I have worked as a birth doula with a lot of Asian ladies who tended to be very small and slim through the hips. In my experience, women tend to grow babies that are just the right size for their bodies to give birth to. I have seen tiny women give birth to enormous babies and big women give birth to small babies. All shapes and sizes!
Obviously it doesn't always work perfectly, but remember that the actual weight of the baby is not really as important as other things like the size of the baby's head, and the position that the baby is in. Being active and upright during birth (rather than lying down) can help to make more room in your pelvis and help the baby get into a good position to be born - and remember too that baby's heads are soft, and designed to mould to fit through your pelvis.
Your doctor or midwife will tell you if they have serious concerns about the baby's size but remember that there is no accurate way to tell the baby's weight before birth -ultrasounds are very innacurate as a way of doing this. And even if a big baby is suspected there is nothing much you can do about it - for example, if you eat less, you might have a less healthy baby (and less healthy you) but you won't necessarily have a smaller baby. It's great that your baby is growing -it shows that he/she is healthy and doing well.
It sounds as if you have some fears/worries about the birth that perhaps are not specifically about your baby's weight but more general. If you have not done so already you might want to consider taking some childbirth preparation classes, or do some reading, to reassure yourself, and to give yourself some ideas for how to make sure the birth goes smoothly. The Birth Book, by Dr Sears is a good and well balanced book.
Good luck! There are lots of helpful people on this list so I am sure you will get some more good information.