Barely any weight gain but big baby? :/

Kaicie
So before getting pregnant I only weighed 160 and I am now about 180. In my opinion to be 34 weeks and have only gained 20 pounds is pretty good. My doctors are telling me that I need to basically go on a diet because baby is measuring 3 weeks ahead. Could he just be long? I watched what I ate before and didn't over eat or anything. Idk what I am supposed to do now.