Question about measuring big..

I've seen people post on here that they are so many weeks but they are measuring 2-3 weeks ahead. If you're measuring ahead does that mean that you might go earlier than your expected due date? Or does it just mean that you're going to have a big baby. Somebody please help me because my OB/GYN doesn't really tell me anything at my appointments. She listens to baby's heart rate and then leaves. Even if I have questions. She tries to avoid them all together. I will wait in the room for 15-30 minutes on her and she comes in there for 2 and then leave.