I don't know if what I say will help, but I hope so. First of all, each pregnancy is different. Even if, generally, a woman will show faster with each pregnancy, it doesn't mean it is a RULE. You know what I mean?
Have you expressed your concern to your doctor? Has your doctor expressed any concern? I think everything will be fine. Like the above poster said, perhaps this baby of yours will be smaller than your other children. I don't think that 16 weeks is TOO far into the pregnancy where you should be so concerned, but that is my opinion. If you still feel this way, talk to your doctor. Are you eating enough calories?
I just want to say that I think you and your sweet joy will be fine!