There are a couple of things to consider. First, babies grow at their own rate. Although there is an average, the nature of averages means that perfectly healthy people fall on either end of the spectrum. That doesn't necessarily mean there is a problem. Second, the technology that doctors use to check growth, especially ultrasound are NOT necessarily reliable. Ultrasound estimates of weight are commonly off by as much as a pound in either direction. They are notoriously unreliable at the end of pregnancy, when baby is getting squashed up in the uterus. Even fundal height measurements can easily be "off" the norm, especially once baby drops down into the lower pelvis. Basically, as his head engages down into your pelvis, more of his body is between your hips and less of it is sticking forward.
It sounds like your baby is checking out very healthy, and it doesn't sound like you have too much to worry about. My biggest concern would be an early induction, "just in case." Inductions carry their own risk, and I would strongly encourage you to do some research about induction, pitocin and the risks of ceasarean section surgery, just in case that possibility presents itself. Unfortunately, many doctors have an underlying mindset that mom's body is a dangerous place for baby to be, and the sooner they can be safely delivered, the better it is. The first risk to baby in an induced birth is prematurity. This has become so common that they have had to coin a phrase for it "iatrogenic (meaning doctor-induced) prematurity." Basically doctors induce birth when they think the pregnancy is far enough along, only to discover that the baby has the symptoms of a preemie, that he was obviously unready to be born. A chemically induced birth is much more painful for mother than a natural birth. An epidural may or may not relieve that pain - many women find it is only partially effective. A chemically induced birth is also MUCH more stressful on your baby. Natural contractions are rhythmical. They start slowly, build in intensity and then come down and you (and your baby) have a break until the next contraction. Pitocin-induced contractions are much harder, faster, and often right on top of each other. Baby is more likely to have a stressed heart-rate, which may in turn lead to a cesarean. Finally, inductions are not always successful. Once your water is broken, you typically have 24 hours to give birth, or again, you are looking at a cesarean section. While C/S certainly have their place in a true emergency, I would warn you to do your own research and be cautious about starting down a path that may well lead to major abdominal surgery.
At this point, the best thing you can do for your baby is to make sure you are eating very well. Eat as many whole foods as you can - lots of whole grains, fruits, veggies and meat. Protein is EXTREMELY important for your baby's growth. Protein is the building block of muscle. Some birth specialists recommend 80-100 grams of protein per day. I ate plenty of lean meat, eggs, and dairy (which also provides calcium). Now is not the time to worry about low-fat options. Your baby also need plenty of healthy fats for her brain development, so think about choosing whole dairy rather than low-fat versions. At the end of the day, when I hadn't met my protein requirements, I often finished my night with a smoothie. I blended milk, fruit and a scoop of protein powder. Good quality protein supplements are available at your local grocery or health store, and can add 20-30 grams of protein in one serving. Just remember that everything you eat is going towards building your baby, and try to choose foods accordingly.
Best of luck,
PS - An antecdote... my best friend only gained a few pounds during her entire pregnancy. She is super-tiny, and she wasn't all that big. Everyone (including her doctor) swore that she would have a small baby. Her doctor estimated about 6 lb at birth. When she delivered, her daugter was 8lb 14oz!