Has she always stayed within her weight percentile? My daughter was in the 25 percentile when she was born. Just a wee little thing, overdue she was 6lbs 12oz and only 18 inches long if you stretched her way out.

She wore newborn diapers for more than 3 months! She weighed around 16 lbs at one year

She has just always, always been a small person.
There were some times I was not comfortable with her weight and talked to my doctor about it. He said she stayed within the same range percentile wise and he was not concerned. She was growing. She just didn't grow fast or big.
I bought a bunch of baby food books (super baby foods come to mind) and made all her food from scratch. Fed her whole grains, etc. So whatever she did get in her, I knew was GOOD food. In researching nutrition for babies/toddlers, I started noticing she was normal. Some days she'd eat like it was going out of style and then she'd slow her eating way down. The eating times were high growth periods. The slowing down times were when she didn't need all that food energy to keep her growing. I started to relax and know that her body was smart.

It is innate that they would know when they are hungry and when they aren't. I didn't want to be
that mom who became a short order cook,
trying to get her to eat whatever she would. I finally came to see that when I cook, she either eats or she doesn't and I cannot fret about it.

I have to let that go. Kids can (not saying anyone here has that issue) use food as a tool of control. They are in control of what foods they eat and how much they eat.