S.,
I can't blame you for worrying! I would too. It;s so easy when anything is happening to you than to your child, no? Things pop in your head that would never if it was you. It's the mama gene :)
I agree with some of the posts that you should just let it go. No more tests! For one, nothing seems to be wrong, and two, you don't want her to get any issues around her body image and weight. Girls have it tough enough.
I'd just continue to offer her good nutrition, even if she only eats small meals, and let time take it's course. I have a friend whose daughter is now 5, similar situation, IUGR, preemie. She's still tiny, but developmentally fine. Her dad is 6'7" and her mom is 5'10"! At least you have genetics with your slight frame partly playing into why she could be small.
One thing that may or may not help you is that she may in fact been preemie. There is a common misconsception in our society that full term is 40 weeks (with our prevelent culture of over doing the indutions and Csections). But in fact, according to Harvard studies, a first time mom's AVG gestation, WITHOUT INTERVENTION (hard to find these days), is actually 41 1/7 weeks. It's a few days longer for subsequent births. This means that if 41+ is the AVERAGE, there have to be births on both sides of that. So a 38 week old could in fact be 4-5 weeks early, and therefor be a preemie (which would explain her low birth weight and current petite size). Who knows. But even if she was a preemie, the good news is she is healthy now.
So focus on that. Enjoy your baby girl and try not to worry. Focus on only the things you can control, like quality time, good food, and lots of snuggles!
A.
ps I sure there are many stories of babies on the flip side who are tubby little guys, even into toddler/ later years, whose parents are lean and fit, and the kids eat well and get exercise. Many grow out of it.