The phrase "largest contributor" refers to the person, organization, or entity that contributes the most or has the biggest impact in a particular context or situation. Full definition
Although advisors don't typically include «behavior coach» in their job description, it's most likely the single largest contributor to the total value they bring their clients. (pagnatokarp.com)
In fact, the study found that it is the second largest contributor after carbon dioxide, trapping more heat than methane, which was previously thought to be second. (ensia.com)