Clothing size is so interesting to me...
I'm 5'6", and I wear a 6-8 (dress vs pants, manufacturer/designer, etc.). I weigh 130lbs. My BMI is 21, and I have a small frame. I say that clothes "fit" if they look nice on me - not baggy around the bustline (usually a huge issue for me), no VPLs, etc.
I met a woman last weekend who recently lost a lot of weight. She announced to everyone that she was now wearing a size 8. I looked at her outfit - a GAP cargo skirt and t-shirt. She was pulling at the skirt all night because it kept riding up on her. The waist was cutting into her belly. I could see the outline of her thong. She determined that she was a size 8 because that was the smallest size that she could button/zip, not because it was the most flattering size for her to wear.
On the other hand, I know women who say that they're a size 8, and their clothes are hanging off - because that's what's comfortable.
It's all subjective.
End of my rant. 