I can empathise completely. My little preemie will be 5 in two weeks time. She was born at 35 weeks but also had IUGR she was 1.26kg (2lb 13oz) at birth.
We were initially told by all the health professionals that she should catch up by the time she was about 3. She is nearly 5 and although she is small to normal height wise (105cm) she only weighs 13.5kg (My 19 month old weighs 13kg!). She is part of a follow up study on low birthweight babies and they are now finding that it takes them much longer to reach a normal
She has always been off the bottom of the weight charts but is very healthy, she eats really well but is like a whippet, just wiry!
Like you we were concerned for a long time but we now realise that so long as she is healthy she is fine, and who nows in 15 years time maybe she will be a supermodel and make millions
unfortunately I think her genes will catch up
One good thing I have now discovered about tiny babies is that they are great for carrying. Until I had our second dd I couldn't understand new motheres complaining about back ache or arm ache carrying their babies, our second has chubby arms and legs as you described and don't my muscles know about it
Hope this has helped, I know sometimes it can feel like you are the only one in the situation.