Why should nodes be small in a concept map?

Question

Cañas, A. J., Novak, J. D., & Reiska, P. (2015). How good is my concept map? Am I a good Cmapper?. Knowledge Management & E-Learning: An International Journal, 7(1), 6-19.

This paper was published in 2015 by the inventors of Concept mapping. In this paper, they have listed 7 Criteria for good concept maps. One of these criteria is:

Concept labels in maps should be only one or a few words labeling a specific concept.

What is the rationale for this criterion? Is there any empirical study on it? I'd appreciate it if you inform me of studies that have tested or explained why nodes should be small.