Every baby is different. If your Dr doesn't have any concerns about baby's weight gain then that's all that matters :)
ultrasounds can be off a little when it comes to measurements. It can be plus or minus a few grams, ounces, pounds etc. They can only do their best to get as close as they can. There are fetal growth charts online you can use to compare but bc every pregnancy is different I would stick to what your Dr tells you.