Growth and development. Not due to vsd.

My 1 year girl was born pre-mature and weighed 1.3 kgs at birth. She was diagnosed with multiple VSD. Her poor weight gain was attributed to this and she underwent a heart surgery at 6 months and it was sucessfully repaired. But even at 1 yr, she is 4.5 kgs only. While she has been teething, she still doesnt roll over or crawl or sit ( even with support ). Though she is otherwise active, responds well to all kind of stimuli, smiles a lot, babbles, uses her feet and hands extensively, has a normal diet, etc. Our pediatrician attributes it to her low weight and says that she will pick up once she gains weight. Though we have complete faith in him, we sometimes get worried if it is some other issue rather than low body weight . And whether the current low weight might also be due to some other reason and not due to VSD.