I'm 5 foot 7 (173.2cm) and my weight is 11 stone (69.9 kilogram / 154.5lb) which is considered medically average and within the healthy BMI. It's up at the high end of the BMI but I'm still slim as it's mostly toned muscle + a fat ass XD
BMI is a faulty system; just go by what makes YOU healthy and happy.
What he said. According to the doctor, I'm underweight... by quite a bit. But, I feel fine the way I am so I'm not worried about it. Even though he's told me to put on a few pounds, I refuse to do so just cause I know I wouldn't feel good if I did.