Round and smooth, well-proportioned structure refers to what calligraphy?

Probably official script. Official script, also called Han Li [1], is a common solemn font in Chinese characters. Its writing effect is slightly wide and flat, with long horizontal painting and short vertical painting, which is rectangular and pays attention to "swallow-tail of silkworm head" and "twists and turns". Lishu originated in the Qin Dynasty and was compiled by Cheng Miao. It reached its peak in the Eastern Han Dynasty, which had a great influence on later calligraphy. Calligraphy is called "Tang Kai of Han Li".