Not necessarily. While putting more weight per inch (really psi...but not in reference to air pressure here) on the contact patch will up the traction available on that tire, it doesn't increase in a linear fashion as weight increases. A wider tire with softer rubber and less psi applied to it can generate more grip and be less prone to slipping than a narrow tire with harder rubber and a higher psi load. So if softer rubber tires are only available in wider sizes that may drive his choice.
Then there is the issue of lateral grip and how the tire resists that...
...and a whole lot of other physics stuff I'm not even going to pretend I understand.