What is the external tightness and internal looseness of calligraphy? Better have an example.

For example, if you look up the copybook, every word "country" is loose, but the space is even. Some calligraphers even write the "Kou" part of this series with one stroke and one circle. But because of the good internal structure, it is beautiful even if it is crowded outside. This is called tight outside and loose inside.