Don't listen to them Stevie! People just comment for comment's sake. As long as your ob is happy, as long as your scans are showing normal growth, then it's all ok. Everyone is different.
I do know how you feel though. I have only put on a small amount of weight too, and 30 weeks with twins! But I have a long body and one of them is so low he's almost in my thigh and my tummy is way smaller than my other preg friends who are only having one and are less preggers than me.
I felt really worried about it for ages. But my ob said it was all just fine.
So tell 'em to stick it!
Jenx