If you lived in upstate New York or southern Vermont, you’d be familiar with the Stewart’s Shops chain of convenience stores. But you might not know just how well the stores’ employees are doing.

Stewart’s launched an ESOP in 2001, and its 5,000-plus employees now own close to 40% of the 355-store company. It recently announced a $17 million contribution to the plan, $6 million more than a year ago. That amounts to nearly 20% of an ESOP participant’s salary, the company  ...