Jimbei bounty was not increased since Marineford war. His accomplishments since then were already huge.
And then, we are ignoring that Luffy had an increase of 1 billion after WCI. And we know, that Jimbei was a major part of it, stalling Big Mom, making the strawhats escape and he was not credit for it.
So maybe with those feats, he already should had been 900 million.