My granddaughter is 15 months and weighs 15 lbs, Anyone with info for me?

her mom was also a tiny baby and child with no ill effects. My granddaughter is eating solid foods and eats well, she has no known health problems and she has undergone a battery of tests, the doctors seem very worried and are going to repeat all the tests they did a few months ago. I only weighed 112 when I had my daughter at 8 mos. She had her daughter at 9 mos and the baby weighed about 6 lbs. I think we are all just small but I still am worried about my granddaughter.Any websites about this?Thanks to everyone who takes the time to answer.