V.A.
On my first, I was 128 lbs and had a tiny pooch at 5 months and didn't really have a belly until I was 7 months along.
My second pregnancy, I started showing at 2 1/2 months. My 3rd pregnancy, I had a flat stomach at 6 months and blew up at 7 months and he was born 8 lbs 2 oz, and my last pregnancy I honestly thought I was carring twins, because I started showing at 2 months and was really big at the end and he only weighed 7 lbs 11 oz, so EVERY pregnancy is different and every woman is different. I wouldn't worry unless their doctor is worried.
By the way, all my pregnancies were "normal" and no problems.
Hope that helps!
V.