Baby looks small for her age. The cause?

Hello, My daughter is 11 months old and weighs 7.3 kg. Her birth weight was 2.7 kg and she was born at 37 weeks by c section. She isn't chubby like other babies and looks small for her age. Many people comment this or have pointed this out, asking whether she's 6 months old. She is reaching all her milestones well and on time otherwise. She is an active and happy baby also and smiles and babbles a lot too. Along with solids, she breastfeeds on demand. Her regular pediatrician isn't concerned and says it's normal, could be genetics also as both me and my husband are petite. But my older child was very chubby and fat at this age. Is her weight less? If yes, please tell how to increase baby's weight? Or any tests that can be done to rule out any deficiencies, if that's the cause? Please help. Thank you.