The best thing to do is find out your body fat percentage, then shoot for a healthy amount body fat. Two people can be the exact same height and weight, but if someone exercises more, or is just naturaly more muscular, they will wear 2 sizes or more smaller and look alot thinner at the same weight.

It also has to deal with porportions too. I have short legs, a longer torso, and very broas shoulders. Because of my shoulders I have to wear a larger size on top then someone with my same measurements chest and waist wise.