Are you deffinate that you only added that single folder?

Right-click the folder in explorer and click properties. The "size on disc" value is how much space the thing actually takes up, and therefore how much DVD space it would need to use. Is this still below about 5.2/5.3gb?

And never try to write too close to a disc's maximum capacity. I try to keep below 700mb "size on disc" on a 730mb CD and 4.3gb "size on disc" on a 4.5gb DVD.