I see your point. Usually it's a two-step process: your breakout role gives you enough attention to land a big star vehicle, which solidifies mainstream appeal.

Portman's breakout role was definitely The Professional. She got a lot of critical attention for that part, and was sortof expected to be a major actress, someday. But you're right that she wasn't actually a mainstream star until Star Wars.

In the case of Knightely, however, her breakout was Beckham, and Pirates was her star vehicle. I don't think most people even know she was in Phantom Menace.