Our garment sizing is given in terms of the finished measurements of the garment, not the body measurement they’re sized for. This means the measurement includes the intended ease of the garment. In our pattern specifications, we’ll always list the finished bust measurement, the intended ease, and the ease as shown on the model (if different—we can’t always find a model who reflects the designer’s intended ease exactly).
If you’re trying to figure out what size will fit you best, you’ll need to add the intended ease as specified in the pattern (or your preferred amount of ease, if you’d prefer more or less) to your bust measurement, and compare it to the sizes listed in the pattern (this also works in the other direction—you can subtract the ease from the finished measurement and use it as a “to fit busts of approximately size X” figure). (Note that for this to work well, you’ll need a trusted companion to measure your bust for you, because this is an almost impossible part of the body to measure on your own.) We strongly recommend that, as a “sanity check” in your sizing, you compare the finished measurements in the pattern to the finished bust and hip circumference of a garment already in your closet that fits similarly to how you want the garment to fit. Comparing garment to garment is often much more predictive, and it gives you a chance to imagine more specifically what the garment will be like on your body at the intended ease.