Its easy for us to say, but really dont worry yourself about it chick. If there was any concerns with the babys health or development they would have picked up on it at the scan. Fundal measurements are known for being inaccurate and should only ever be used as a guideline, not a definite. Same goes for scans, they can estimate the length and weight of your baby, but are not 100% correct.
I have had the opposite problem and my baby is always measured at about 4 weeks ahead, but after further investigation and growth scan etc, was told I prob will have a slightly bigger than average baby but certainly not a whopper. They estimated her weight to be 4lb 2oz at 30 weeks (the average is 3lb), but also gave a 25% margin of error so this could be wildly different to her actual weight. After a bit of panic when they first told me she was big, Im now not worried as everything is going well and thats what ultimately matters. My first baby was bigger than average at 8lb 14.5oz, so it stands to reason this one will be similar, Im just destined to carry slightly bigger babies. Must be my child-bearing hips!!
On a similar note, a good friend of mine just had her baby about 10 days ago, and all the way through they told her that her baby was small for dates. But she is only small herself, so its no real surprise. Her baby was smaller than average at 5lb 10oz, but is perfectly healthy, so again nothing to worry about. At the end of the day, your body will only grow a baby big enough that you can cope with. Whether this means small or large is irrelevant as long as its healthy and development is normal, so please dont worry.