First, because the material of hard pen tip is hard, its line change is not as big as that of soft pen;
Second, because the commonly used hard pens are mainly pens, ballpoint pens and pencils. The lines are much more delicate than the brush, so the font is much smaller, even smaller than the fine print of the brush;
Third, due to the material of the pen, the hard pen is easier to control than the brush.
Hard pen calligraphy is divided into hard pen calligraphy art and practical hard pen calligraphy. The former mainly emphasizes artistry, while the latter mainly emphasizes practicality. The artistry of hard-pen calligraphy mainly expresses the writer's understanding and feeling of calligraphy art through the changes of lines and structures, and it pursues beauty, emotion, interest, artistic conception, personality and so on;
Practical hard pen calligraphy, that is, daily writing, is mainly to meet the needs of practicality and facilitate writing and identification. The key is to make the writing fast and standardized, so the practical hard pen calligraphy is mainly running script.
References:
Baidu encyclopedia-hard pen calligraphy