One-third neutral and full neutral
In single phase circuits, the neutral conductor carries all the return current of the phases, and must be sized appropriately.

In a three phase circuit, the neutral conductor instead carries the imbalance current of the phases and third harmonics, which are in phase with each other. In a well-balanced system, it is possible to derate the neutral conductor on the assumption that it will be carrying considerably less current (because a balanced three phase system, ideally, has no need for a neutral). However, this must be done with care, because if the system becomes unbalanced or the harmonics get out of hand, you will quickly exceed the rating of the neutral conductor.

The "difference in structure" is merely a difference in the size (area) of the neutral conductor.