I've found that different brands make different sizes... And being top heavy was really embarassing until I discovered a store that caters to D cups and up
While they don't have the store where I live now, I order online
I would suggest you try on a few bra's and see which ones fit best. Having said that, some bra styles just don't work on certain boobs.
I've found that most cheapr brands didn't even cater to my previous size (38E, UK sizing) and by boobs we always busting out. I've found that VS bra's were more sexy than practical and didn't really cater to the large, heavy breasted woman
I only realized a few years ago that it was actually possible to find a bra that has the wires sitting directly on the skin, under the boob! For several years I just thought the bra was supposed to sit "on the boob"
That and I'd get really bad shoulder pain from the straps. These days the band holds most of the load