Size difference?

We are expecting di/di b/g twins in November. We are 26weeks. At our scan today, baby a (girl) is measuring in the 33rd percentile at 1lb 12oz. Baby b (boy) is measuring in the 67th percentile at 2lb 5oz. The doctor said if baby a drops much lower in percentile we would need to expect to have extra monitoring and it could also sometimes mean a chromosomal defect. Has anyone else experienced this big of a size difference? I know it could be because ones a boy and ones a girl, and boys tend to be bigger, we just went through a lot to get these babies and it makes me extremely nervous!