How does boston proper fit? Is an XS really for someone who is usually a 2 or 4?

I have never ordered their clothes before. I wear a 2 in pants. Most of my tops are size small, sometimes xs, but the measurements given on BP's size chart puts me at larger end of the small. I read reviews on some of the items I want to buy and I may need xs in pants, but the sizing is quite vague. Can someone help?