P.M.
My daughter started off at a 'normal' weight of 7.11 After the first month, she quickly drifted down to the 5th percentile in weight but average for height and head. She is now a beautiful 5 y/o, 42" (normal) and only 35lbs. She's so willowy, elfin and petite... but very healthy and proportionate.
Every child is different and with the childhood obesity and diabetes epidemics in America, I'd not be worried about her being on the small side. If her eyes are sunken, skin is dry, when you pinch the skin on her wrist it stays pinched, and she's not having many wet diapers - THEN I'd worry about.