My SIL is a rail. She's so thin! And with each of ther three pregnancies, she's gained too much, 40-50 lbs. It's just what her body does. She's stopped worrying about it. She eats pretty healthy, does her best to provide good nutrition for her growing baby, and ignores the doctors on that point.
As far as I know, big babies aren't a product of over-eating. It's just how they grow. The reason they ask you if you've had a baby over 9 lbs is because of gestational diabetes. If you have GD, your blood is full of sugar, which goes to your baby. The baby puts on lots of fat. A GD baby can really fat, to the point of the eyes being just puffy slits.
I had a 9 lb baby at 41 wks who was skinny as a bean pole. They took my second one by c-section at 39 weeks and he was almost 9 lbs (they thought from a late u/s that he'd be 10 lbs by his due date, so they said sorry, you can't try a VBAC) With both of them, I gained exactly the right amount of weight. My second is still just huge at age 3, over 97th percentile for height. My first is 75th percentile for height (they're both lower in weight percentiles--they're not fat!)
My friend also gained the right amount of weight. She'd had 4 average-sized babies, but her 5th weight 10 lbs! It just depends on the baby.
I'd say don't worry about it.