Read the excerpt from Levitt and Dubner's Freakonomics
Driving around the parks that encircle Washington, he solicited customers with a simple pitch: early in the morning,
he would deliver some bagels and a cash basket to company's snack room; he would return before lunch to pick up
the money and the leftovers. It was an honor-system commerce scheme, and it worked. Within a few years, Feldman
was delivering 8,400 bagels a week to 140 companies and earning as much as he had ever made as a research
analyst. He had thrown off the shackles of cubicle life and made himself happy.
The authors prove Feldman's success by describing

The authors prove Feldman's success by describing the size of his business.

Explanation:

At the end of the excerpt, the authors talk of how Feldman threw off the "shackles of cubicle life". He went from being an employee in a cubicle to being a successful self-employed man. To prove his success, the authors provide us with numbers that show the size of his business:

Within a few years, Feldman  was delivering 8,400 bagels a week to 140 companies and earning as much as he had ever made as a research  analyst.

Being able to deliver that amount of bagels to that number of companies can only mean his business is big. He'd need to have several people working under him as well as a quite decently sized infrastructure to do it.