The measurements to determine if you are overweight are taken from what is now called BMI or body mass index. Body mass index takes your height in inches and weight in pounds and puts them into a simple formula and then uses the number on a chart to determine where you fall according to the government. Scoring an 18 or less, you would be considered unhealthy and underweight. A measure of about 23-26 is average and anything above 26v in considered unhealthy. The BMI would be a very useful tool if everyone were built the same way, however everybody is different.
Some people naturally have more muscles than others and are considered overweight for it.
Others are just naturally skinny and don't have any other choice than to be like that.