In The Twilight Saga: Breaking Dawn Part 2, why did Bella become more powerful when she became a vampire? Even Edward, Rosalie and Jasper were humans who later became vampires and no doubt they all are powerful but Bella is much more powerful than them. Why?
Bella, as a human being has an extraordinary shield power which makes her immune to Edward & Jane's mind power.
It is mentioned in the movie, I think by Alice, that a vampire's powers are an enhanced version of the qualities they had as a human. So as a Vampire, Bella's powers of shield are enhanced & she can project that power outward to others minds as well.
If however you are asking about her strength as a vampire overpowering Edward, that strength boost is temporary for a new born Vampire as seen in the movie Eclipse. I don't remember the exact reason but something to do with hormonal changes as a new born vampire.