I have no idea how much these sites are earning from Amazon but I believe each earns at least $100K a month?
To other people who are saying Amazon offers a low percentage, I suggest you read one of Darren Rowse's posts about his Amazon earnings. This is actually the article that encouraged me to start using Amazon a year ago.
Although right now my main source is Clickbank, I have more hope in Amazon as the earning potential in promoting physical products is higher and long-term that promoting digital products.
Do you know any bigger sites that promotes Amazon products? Let me know and we'll take a look to analyze their strategies.