20 years ago, she'd have been right on track. She wouldn't be close to puberty/menstruation at all. The age for breast development & other sex characteristics and menarche has constantly dropped. So your daughter seems behind just because everyone else is doing it sooner. She's growing consistently, she's eating well (and maybe she's not eating a lot of foods with hormones so she's not getting "artificial" breasts and so on?), and the doctor aren't worried.
My son was the shortest but also the oldest in his class (we didn't start him in kindergarten until age 6 - lots of reasons, all turned out to be good). He didn't kick in with growth and hormones until 9th grade. He started to run track and at the same time his body started to change. He was also low on the height charts and consequently on the weight charts (although his weight was proportional to his height). The doctor thought he would be 5'6" or so, but he's easily 5'9" now and was by senior year in high school. He was very "late" in getting body hair and he still doesn't have much of a beard (which is okay by him - less shaving).
I'd just let her be petite. You are, and you've turned out okay right? And really, what are you going to do about it? There's clearly nothing really seriously wrong and her growth isn't stunted, so she's just petite or she will get her growth later on. While the other girls are feeling gawky, your daughter is doing fine.
I think, in general, we put way too much emphasis on body image, and then we wonder why our daughters (especially) are so hard on themselves as teens, never being happy with who and how they are.
Your daughter will come along in her own good time. Don't fill her up with food to get her to gain weight - she will develop an unhealthy relationship with food. Continue to feed her good things (and allow some treats too), and be sure she has the chance to have a few snacks during the day. Small people sometimes don't stay full that easily - but truthfully, all of us should eat less at each sitting and have at least 2 snacks during the day. Be sure she has a mix of protein, healthy fats, and complex carbs, and let her be.
I'd give her "the talk" about menstruation and body changes (and, yes, sexuality and peer pressure and boys and feelings), and reinforce it in small doses over the next few years and into her teens, so that she is prepared for what happens to her and for what she hears about from peers. But let her know that everyone catches up eventually - she's going to be "late" in some things and "early" in others, just like kids who walk, talk, potty train and sleep through the night do it differently from the next kid. Have her prepared, but not feeling frustrated or impatient or, worse, imperfect.