Well, he was given the option to put up some money with Jeff at the beginning and declined. I'm guessing that would have doubled his eventual stake...

It was all done on a threadbare budget. At first Bezos backed the company himself with $10,000 in cash, and over the next sixteen months, he would finance the startup with an additional $84,000 in interest-free loans, according to public documents. Kaphan’s contract required him to commit to buying $5,000 of stock upon joining the company. He passed on the option to buy an additional $20,000 in shares, since he was already taking a 50 percent pay cut to work at the startup and would, like Bezos, earn only $64,000 a year. “The whole thing seemed pretty iffy at that stage,” says Kaphan, who some consider an Amazon cofounder. “There wasn’t really anything except for a guy with a barking laugh building desks out of doors in his converted garage, just like he’d seen in my Santa Cruz home office. I was taking a big risk by moving and accepting a low salary and so even though I had some savings, I didn’t feel comfortable committing more than I did.”

(From "The Everything Store")