I'm only guessing here about how the media are publicizing/discussing the film, but I think that in general, folks like to capitalize on the partnership among Tim, Johnny and Helena. They've done a bunch of films together now, but Helena has not always had a starring role- for instance in Charlie and the Chocolate Factory, her role as Charlie's mother was actually quite small, and even in Alice In Wonderland, I think she was billed as supporting actress. It was only Sweeney Todd and Corpse Bride where she was really a lead. But I think the media like to emphasize her in Burton/Depp films, anyway, just because she's perceived as part of the package
All to say that while I wouldn't be surprised if Julia's role is bigger than Helena made it sound (if so, perhaps she was being sarcastic but I'd kinda take it as modesty), I also wouldn't be surprised if it turns out more of a supporting role. But I guess we just have to see!