I wouldn't be too concerned. My girl was like that. We couldn't keep the weight on her and she ate like a horse! She's very tall and lanky but very slim, and was eating about 6c of high protein food. She is probably just a bit taller than standard, or at least is at the high end, and was between 60 and 65 lbs until she turned 4. Then she gained a few and didn't need so much food. Now she's almost 6 and looks great! You can see the last rib, but not all the others. She's eating between 3-4c per day and weighs between 70 and 75 lbs - has been holding that for about a year and a half.