Growth and Immunity Issue in child. Age.

Please Advis. My daughter is 6 now, but her weight has reached a maximum of 14 kg and height 107cms  with a current BMI of 12.06. Her regular pediatrician says she is in the height range of parents and weighs accurate as per her height and we shouldn't give her growth medicines and spoil her long term health. We did consult other pediatrician who said there doesn't seem any systemic illness, it might be due to growth hormone, but we will not be able to do anything about it. Me and my husband are short, height is not my big concern but weight is drastically low. She looks all bones and weak, but is very active and good at studies too. Endocrinologist too is simply giving a new blood test everytime and she said it might be due to vitamin D deficiency. I don't know if I should proceed with investigating the cause for her low weight and height or just come to a conclusion that she will be fine. My elder one had a normal BMI with 4th and 15th percentile of height and weight for this age.