My 6-month-old weighs 15 pounds and she doesn't have much upper body strength. It's most noticeable when she is in her walker. Is this normal for her age and weight?
Most girls that age weigh between 12.5 and 19 pounds, so her weight is right in the middle of the range (assuming she has a normal length and head size).
Kids tend to develop best when not in a walker. While the walker is fun for them, it strengthens the calf muscles before the thighs and can decrease coordination. On average, kids learn to sit well on their own, not in a walker, at around 6 months; but there is a range where this is normal. If you are concerned, her doctor can get a good sense of her strength and development at the six-month visit.
In the meantime, stop using the walker and that will probably increase her strength and development. Give her plenty of “tummy time“—supervised periods of time on the floor on her stomach. She will most likely use her upper body more, thus building upper body strength and coordination.. This is one of the reasons that the American Academy of Pediatrics no longer recommends walkers.