What makes a dessert rich?

Most of us have tasted desserts that we would call rich. It only takes a tiny bit of a rich dessert to satiate your sweet tooth. While the dessert is delightful, the thought of eating very much of it is almost repulsive. Rich desserts are distinctive; it only takes one bite to recognize their richness.

I am curious, because the richness of a dessert does not seem to depend on what it is. For instance, some cheesecakes are much richer than others. Some brownies are rich, while others are not rich at all.

I am not asking what it means for a dessert to be rich; I can recognize that well enough. Rather, what it is about a dessert that causes it to be rich? Even within the same type of dessert, what makes one richer than another?