Most water runs off between the soil and the side of the pot, thus haveing a small pot with the roots just touching the sides a bit gives the plant optimum access to it. But if roots start crossing each other by twisting around the sides, they choke each other... so we pot up....
In my experience potting up to larger size brings faster growth than plonking a seed in the middle of a 30gal... At the end of the day though the plant is limited by the container's size.
i agree with that