There are a variety of synonyms for the phrase "decreases by." Some options include diminishes, drops, reduces, lowers, lessens, declines, dwindles, shrinks, contracts, and abates. Each of these words conveys a sense of decreasing in size, quantity, or intensity. For example, if the price of a product goes down, one could say it diminished or reduced. If a person's interest in a topic wanes, it could be said that their attention dwindled or that their enthusiasm contracted. Understanding the nuances of these synonyms can help writers and speakers more accurately convey their intended meaning.