The Dharma is something that cleanses the mind [i.e., purifies the heart] so that it's bright, clean, and happy.
People differ in their temperaments: Some are crude, some intermediate, and others refined. This is why the Buddha elaborated on the Dharma in various ways in line with the character of his listeners.
In other words, he took short things and explained them until they were long. For example,
- sometimes he'd explain the good results of generosity,
- sometimes the good results of virtue, and
- sometimes the good results of polishing the heart: meditation.