I don't think you're giving enough importance to what being 100 pounds overweight can do to an athlete, even if he's not yet 30 years old. "Even with his large size" is just a terrible beginning to any sentence trying to defend the Sandoval signing.
I liked Ben, but this was a terrible signing right off the bat. Anyone could see it. Decline, problems with body type, questions about his work ethic, the whole shebang. Humans make mistakes. Ben was not some sort of deity impervious to errors. Signing Sandoval was a dumb idea right off the bat, considering he already had a better overall 3B and needs in the rotation.