Guerrero's weight is absolutely a problem for a 20 year old kid with a history of injuries. Young Tony Gwynn was lean, stole 50 bases, and stayed in the outfield for his entire career. Fielder and Vaughn were fatter than Vlad, but they were first basemen with no defensive value and were finished as elite players by age 30.
Troy Glaus and Chipper Jones were huge third basemen who stayed at the position, but they were built like tanks and quick enough to cover shortstop. Guerrero is one undisciplined winter away from looking like Pablo Sandoval or Kendrys Morales.
I think we can agree that Vlad is a generational asset that could be worth $100 million in surplus value over the life of his controllable years. If upper management hasn't hired a nutritionist to count every single calorie that goes into his body 24 hours a day, 7 days a week, they need to pick up the phone and do so today.