This illustrates a general principle that has much wider application in economics. In economics, we speak of a law or principle of diminishing marginal utility.
The "Law of Diminishing Marginal Utility" states that for any good or service, the marginal utility of that good or service decreases as the quantity of the good increases, ceteris paribus. In other words, total utility increases more and more slowly as the quantity consumed increases.
This is "diminishing returns" from the viewpoint of the consumer, and is a general principle of economics. There might be a threshold before the principle applies. For example, the marginal utility of golf clubs might increase until you have a fairly full set. But beyond some threshold, marginal utility will diminish with increasing consumption of any good.
As we will see, there are other applications of "diminishing (marginal) returns" in other branches of microeconomics.