That's not because Bellamy can't play for a big club. Blackburn was the only season he ever scored like that, his Liverpool record was no different than his Newcastle one really. He just had a good season, secured a move to a top club, then went back to his usual form.
Bent's a quality player, he has everything he needs. He's fast, he's a great header of the ball, he scores with his right and left foot, he's a great finisher, he's a proven Premiership goalscorer. This isn't a one off Bellamy or a never-proven-himself in the Premiership Kuyt. At a top club, playing regularly, he'll get better service and spend more time in attack than at Charlton, and will score at least as many, no doubt.