So this top is padded. Really, really padded. In addition to being pretty strangely shaped, the padding is also not removable, which would make it an excellent candidate for an impromptu lifesaving device in the water. (Not actually, but you get my drift.)
I'm a 34 C, usually a Medium in tops, and ordered a Large. It fit fine, minus the concerns noted above.
I ended up exchanging this for the Triangle Top with the same print, also in a Large (http://www.backcountry.com/volcom-notaho-triangle-bikini-top-womens) and loved it.