Actually, the biggest single charitable givers, in terms of absolute dollars, are atheists. Bill and Melinda Gates and Warren Buffet, who have given tens of billions of dollars in charitable contributions, are both atheists.
The problem is, weekly contributions to churches is counted among charitable contributions, which puts the religious ahead in mean terms, simply because they feel obligated to drop something in the collection plate on Sunday. I don't consider most of that money to be charitable as much of it goes to church upkeep and the rest, which may ostensibly go toward helping people, does so at the cost of being a religious outreach. They put up soup kitchens, not necessarily to help people, but to try to convert them. The food is a means to get the poor in the door.
Further, atheists don't tend to stand up and wave their arms and scream "we're atheists!" when giving. They just give. They're not out for notoriety. Things have started to change, there are more openly atheist groups giving money to Doctors Without Borders, for instance. In fact, the reason many atheists haven't given more openly is that many charities have been so demonstrably religious, pushing their mind-poison on people in the guise of helping them. I wouldn't give any money to those groups either. But luckily, some of the largest charitable groups out there today are entirely secular, such as the Red Cross, which removes that particular obstacle.
To be honest, I consider atheist giving to be much more valid than theist giving, simply because atheists give, simply to help people. Theists give, at least in some part, to make nice with their imaginary friend in the sky. Atheists get nothing out of it but the satisfaction of giving. Theists, while in actuality get nothing out of it, are certainly driven in at least some small way by scoring points.
