Weight gain

Any mummies experiencing your LO not gaining weight according to their age? My LO is 7 months, weighing 6.1kg. She’s on the smaller side since birth but not premature. Drank well, eat solids well, not picky, pees and poo well. Height and head size is growing as per normal. Any tips to make LO gain weight? So tired of ppl saying she’s small. Parents are petite size.