9 month old

So my daughter has been small since birth. Barley 6 pounds. She’s been consistently in the lower percentile for her weight and everyone always says “wow shes tiny!” “She’s so small” Or compare their babies weight to hers...... it’s very frustrating. I feel like I’m doing something wrong or like something is wrong with my milk? She hits all her developmental milestones and her doctor has never seemed concerned. Is there anything I can do to help her gain weight?.. she it’s consistently every 3 hours. 4-5 oz at a time even through the night.