I had to go up a cup size in the Tango plunge style (which I am wearing today, coincidentally). Try a 34E or 34F (I don't know off hand whether a size larger than a DD is an E or F).
You might also give the balconette style a try. Try a DD and an E/F.
As I said, it's hard to know what is going on when you are only looking at one bra in isolation. That's why I recommended trying a variety of styles and brands. If they were ALL giving you armpit fat, it would indicate that you need to try a bigger cup size. If it's just this one, it could be that it runs small, that it isn't the right size to begin with, or that it isn't the right style or brand for you. But there is just no way to know.
