The more pregnancies you have, it's likey the earlier you will show. As far as your breast are concern, I don't think you have anything to worry about. If they get too sore, try using a warm compress (towel or washcloth).
Each pregnancy is different so that is probably what it is. I would say as far as being bigger than what you should be it could just be you are showing sooner because you already had babies. Did your OB check your HCG levels yet? I am 14 weeks right now and started showing around week 9 or so and this is my first pregnancy. I think I started to show early because I have a tilted uterus. Do you know if you have a tilted uterus? Congratulations on the pregnancy!!!!!