Brenda,
As the mother of a 10% in weight baby boy, I understand how you feel. Jacob's doctor (who I trust 100%) asked if he was a good eater - and he's an eating machine. We just gave him very healthy foods and the doctor said that was part of it. She'd "rather that than an overweight baby chowing down on french fries every meal."
I agree with the others that some kids are smaller. BUT, you need to decide if you trust your doctor. Our son had jaundice when he was born which resulted in many blood tests in his first weeks on Earth. He had HORRIBLE colic and fevers at about 2 months that resulted in an ER trip which had more blood tests and urine test (catheter - ouch!). We were even sent to Lutheran General for a possible spinal tap at one point because he was absolutely inconsolable at the doctors office. (Luckily, we didn't need to do that). He had many many ear infections that resulted in tubes in his ears.
What I'm saying is not once did I question anything my doctor wanted to do. And, I'm not a pushover by any means either...trust me. Our doctor had a detailed explanation for everything that we came across and was with us to work through it all. I never felt that she was trying to run unnecessary tests or anything like that.
IF you feel hesitant about your doctors opinions, it's time for a second one. A lot of children are smaller than "average" and doctors don't see an issue with it because there isn't one. But, if you've been taking your child to the same doctor her whole life, the doctor may have other reasons for wanting tests. You know what I mean? If you're not sure, take her elsewhere to get looked at.
There are a lot of good peditricians out there, but there are some questionable ones, too. I'M NOT SAYING YOUR DOCTOR IS, but if your "Mom Instincts" are on alert...maybe that's something to think about?
Best of luck.
T.