When learning calligraphy, why do you say that a big pen can write small words, but a small pen must not write big words?

Because writing often requires one-third of the nib, the pen has the best elasticity and writes smoothly. No more than half of the written part. A small pen can write big characters, for example, most or even all of the brush heads. Will seriously damage the elasticity of the brush, many strokes can not be used, many skills can not be reflected. The written words are hard to look at.