Question about "Living Green" and flying Coach?

Lately I’ve been interested in the trend towards living a more "green" lifestyle and have been reading articles, reports and such. In many of these reports, there are references to travellers flying Coach instead of Business or First Class on commercial flights in order to be more "green". My question is: What is the connection between flying Coach and being more green? How does paying less money for airfare reduce pollutants and emmissions? Or am I missing a bigger picture here?