Most small-chested women I know tend to wind up with larger breasts after pregnancy. They swell up during the pregnancy and then settle back down somewhere smaller, but still larger than they were before the pregnancy. Around here we call it "getting a visit from the titty fairy."
The second pregnancy, in my experience, tends to just make them sag. I'm working off a limited sample size (women I've known through multiple pregnancies that let me see them topless, so less than ten), so that might not carry over to the majority of women.
I guess the moral of that story is that I need to meet more women that let me see them topless. You know, for science.