My SO finds it much easier to get dresses to fit, than two piece outfits or even pyjama sets, which I think is common for CDers. She has to buy a top that is one or two sizes larger than the bottom, even when she wears her breast forms and hip pads, and this is to accommodate her shoulders. If she didn't wear any breast forms though, even the dresses wouldn't hang correctly. And she is not heavy.
With the dresses, I think that clothing manufacturers must accommodate a wide variety of women's shapes and heights, and many are designed to be fairly flexible. I personally prefer a tailored fit and so there are a few brand names that I try to stick to that I know will fit me like a glove. If the dress is tailored I have a hard time finding one that is long bodied enough (the waist of the dress is always too high on me) and I fare better when I find sizes for tall women, same as in one piece bathing suits.