You need to fill in the cup in order to hold the gown up. Base your sizing off of the waist measurement. There’s not a lot of wiggle room in strapless styles, they’re made to be snug! If you go a size down, and your waist is a bit bigger than the listed measurement, there is the possibility you’ll have some issues zipping the dress up. It is always better to go larger and have it taken in than hope to have enough fabric to let it out!