Yeah, because baseball totally has a history of fat players sucking. David Wells, Mo Vaughn, CC Sabathia, Both Fielders at varying points in their careers, Bartolo Colon, Dmitri Young, Pablo Sandoval, Jonathan Broxton, Babe frigging Ruth.
Maybe they need to get into better shape with regards to cardio or something like that, but let's not pretend that them being massive guys is some automatic death knell. Pitchers and 1Bs have always historically been able to perform even if they're heavy because the weight doesn't have an impact on them the way it would at other positions. Now Kirk, on the other hand... Though in his case it's less about him being less effective because of his weight and more that he's gonna murder his knees and be moved off the position sooner if he continues to carry such a heavy frame.