I've seen her in person. She's very thin, so I'm betting her ribcage is pretty small. If that's the case, having D's relative to her ribcage won't make her boobs overly big. Cup is always relative to underbust & overbust measurements. I'd be surprised if her ribcage is bigger than 28" or 30".
Besides, "D" isn't that big to begin with. They're a start in the right direction.