Not technically, but depending on how you want to think about it, practically yes.
The hitbox has a duration during which it is active. If the hitbox is moving during this time, a larger area will at some point be hit by the hitbox.
As Reinhardt, you can spin in a quick circle during the middle of his swing, and the hitbox will spin with him. Technically, this means you move the hitbox away from where it started. If you were to hold still, the hit box is active in its original position for its entire duration. Enemies moving into the hitbox near the end of the duration will be hit. By moving it, it is no longer in its original position (enemies moving into that position will not be hit) but it travels in an arc around Reinhardt (staying in front of him) and can hit many enemies.
From a practical standpoint, you can spin Reinhardt faster than enemies would move into your hammer's hitbox, and the rate at which enemies enter into the hitbox's original position is relatively slow, so you basically hit more enemies this way.
Ideally, you will spin the hitbox if there are characters spread around you. If there's only one character you're trying to hit and you're heading towards each other, holding the hitbox in place (or pushing it forward as you continue to move forward) may make it easier to hit somebody in front of you.
You can test this out in the training grounds. Stand near the two bots in the front and start your swing while positioning Reinhardt's body in various different initial angles relative to the bots. Try standing still vs slow spinning vs fast spinning. After the attack, check the health of the bots to see which ones you hit.