Here is a study done by fangraphs on heavy vs. average players. The data absolutely backs up my point. We can't just disregard the data when it does not suit one's point. It's mostly focused on Prince Fielder, but it gets the point across.
http://www.fangraphs.com/blogs/how-will-prince-fielder-age/
http://cdn.fangraphs.com/blogs/wp-content/uploads/2011/10/Curve2.jpg
For those who don't want to read, look at the curve. In general, fat players peak early then start a sustained decline around their mid-20's, sometimes falling off a cliff around their late 20's, which is exactly what's happening with Sandoval. The fact that the weight clause was a sticking point in negotiations should have been the red flag that sent the Sox far, far away from him. Then they go out and sign a better player and put him in an unfamliliar postition. Stupid all around.
Another article, specifically on Sandoval:
http://www.fangraphs.com/blogs/which-pablo-sandoval-did-the-red-sox-buy/