i ordered a M with a 40" bust and 35.5" underbust, and the fit is tight & secure. it's very comfy, and i also suspect i could have sized up & still had it fit, though not sure since i haven't tried a large, and i haven't worn this one enough to see how it "relaxes" with wear. i like that the becky is reversible & it looks great both ways! the fabric also feels really solid & high quality in a way that many of my similar bralettes don't.