I'm 5'6" & 125lbs with a somewhat muscular build and the medium fits me perfectly.  This top is very flattering and has great support.  You could almost where it to dinner if you wanted.  I bought three tops:  purple, blue and black and they are great colors.  The only complaint is there is a tiny amount of gaping when in downdog that shows a bit more skin than I usually like but if I were bigger breasted, I probably would have this issue.