with a piece of popular music you get a single ethos, a single feeling,a single mood associated with the piece, and the piece will tend to be shorter.

在流行乐里，一首歌只表达一个中心思想，一种感觉，一种情绪，所以这首作品也就会短小一些。

耶鲁公开课 - 聆听音乐课程节选