Double congrats to you. I have 11 month old B/G twins and we too were completely shocked and they are a definitely a handful, but it’s wonderful. They are so close even at 11 months I can tell they really love being with each other. As for your question, since they are in two different sacs they are probably but not necessarily fraternal twins so it’s totally normal for them to be different sizes. I won’t go into the details about the different days the egg can split verse the release of two eggs, but if you’re interested just Google twins pregnancies and you'll get a million charts that will explain this. Regardless of them being identical or fraternal, there are a million different reasons why they can growing at different rates, implantation site, the specific age of the each fetus, the specific genes each fetus inherited, gender, etc. The point is each baby is different so as long as they are growing at relatively the same rate everything is fine even if they are different sizes. You’ll probably be referred to a high risk specialist who will do tons of ultrasounds during your pregnancy to monitor the babies growth including the size of the brain, head circumference, heart, and bone lengths, and these are better indicated of growth so don’t worry too much about the appearance of a size discrepancy during an ultrasound at 9 weeks. As for the comment below about the “twin to twin transfusion”, that is ONLY a concern for identical twins in the SAME sac or different sacs but SHARING a placenta (very rare) so you probably don’t fall into this category. I hope that helps put you at ease, but I’m sure you’ll still stress during your whole pregnancy, I know I did. I know people have said to join groups for multiples and I would never discourage anyone from looking into this, but I thought they were a complete waste of what little time I had because almost all the advice was common sense so it wasn’t really for me. Congrats again and good luck.