If she's happy and busy, she's just right for her.
I have one like that, but she was 9lb 5 oz at birth.
and over 23 inches long. At 3 months she was only 14 lbs, didn't break 20 lbs till a year old and remained short. But she talked at 9 months and abandonned diapers at 18 months. can't tell me she was underdevelopped. Today at 6 she is a whopping 42 lbs, but is relaly long and lean, just a few inches shorter than her 9 yo brother.
: Healthy active kid. She asks how come her younger sister weighs more, i tell her that's just how it is, the she is just right for who she is, and her sister is growing the way she is supposed to, as is her brother.
They are all different, some are tiny, some are huge, others are slender and others chunky. I think as long as they grow, and aren't being pushed to te overweight category, they are fine.