Dont' worry, my twins are also one big one small (born at 38W: 2.5kg/ 2.2kg) and now happy and healthy!! (six months: 9kg each) They were only around 1.6kg during W28 and I was worried too, but suddenly gained weight afterwards until they were born.
BBs will grow a lot during the last two months of your pregnancy but make sure you get enough sleep. Sleeping and resting is the most important thing to make the baby grow. Also, your belly will become very big too, so lying down and walking might not be easy for you after W30.