He could just be a massive human being. Those exist too. Fielder's conditioning has improved greatly as he's been in the Majors. It's not as simple as "wow do you see how big he is? He's FAT (sorry, "excessively overweight")." Fielder doesn't have fat guy/old player skills at the plate (he doesn't K enough, and he hits for good BA), which IMO is a huge deal for his future and how he ages. It even kind of speaks to the shape he keeps himself in, as logically someone as excessively overweight as him would see some kind of decline somewhere in season (BA might fall due to the bat being slow down due to being fatigued, K's might go up, something)...None of that shows up.