It's not like he's fat though. If you've ever seen him, he's lean and in excellent shape.
If you take someone like Bautista, not nearly as athletic as Heyward but still manages to keep himself in great shape. Now, give Bautista Heyward's defensive instincts (reads/jumps, efficient routes) and you have a fantastic defender. Heyward isn't elite just because he's fast, but he covers so much ground in part because he reads the ball off the bat well and takes great routes to the baseball.