Hi, H.!
My oldest, now a beautiful five-year-old girl, has always been a light eater. She was breastfed until 15 months, and even as a newborn she ate frequently in small amounts. My doctor put us through the wringer about "failure to thrive" when she was about six weeks old, so I changed doctors.
My current doctor (and I am so happy with him!) has two thoughts about weight. One, as long as they maintain their place on the weight curve, they're fine. (In other words, if her weight has always been around the 15th percentile, it's fine for her to stay in the 15th percentile.)His second point is that they form their permanent fat cells early in life, so he'd rather see kids on the thin side than the heavy side.
Just to give you an example, my daughter was 18.5 lbs and 32 inches at 18 months--that's 25th and 70th percentiles. By 24 months she was 22 lbs and 34 inches--57th and 60th percentiles. So you can see that she was tall and skinny, and then totally balanced six months later. And I didn't change a thing! That's just how she grows.
Keep in mind, that was before the World Health Organization came out with the new percentile charts for breastfed babies. They do NOT gain the same way that formula-fed babies do. You might want to look up the WHO charts for breastfed babies on the internet.
As for eating, NOW my daughter will eat 4-5 bites of each thing at a meal, but when she was 18 months, I was happy if she ate three bites of each thing in her. We just kept serving balanced meals (what we eat). She loves fruit and yogurt, bread and pasta, so I made sure she got those. She's learning to like meat and veggies.
I still stress about her getting enough protein, but I try not to show it. She's old enough to know that she has to eat her five bites of everything. My two year old, on the other hand....we serve healthy food and he eats what he chooses. He's not old enough to understand.
No child, given good food to choose from, has ever starved themselves. Try not to pressure her, and make sure you're comfortable with WHY your doctors want her to gain.
Good luck!