The Sox didn't sign Panda to play Gold Glove defense. They know what they are getting in him defensively. It's not like he completely stinks defensively. If the loss of 30-40 pounds is going to result in a loss of power, then any gain that he sees defensively would probably not offset the offensive loss.
I agree with Jacko as well - Pablo is not likely to age well. Although I'm excited to have him on the team, I was not crazy about his signing either. I would have preferred Headley. That said, my point was that it's not like it's any surprise that he's fat. That's what he is, everybody knew this going in, so why all the media attention on how big he is, like it's new news or something?