According to wikipedia, the Falcon 9 1.0 had ca. 10t payload capacity to LEO. The Dragon has a 3.3t payload capacity to the ISS with a dry mass of 4.2t that leaves 2.5t unused lift capacity.
Is this all used for fuel and propellant? That seems like a lot (that would mean more than 800m/s delta-v assuming the Draco has 300s isp, as it is "comparable" to the R-4D). The difference grows even larger with the Falcon 9 1.1 and (presumably) the full thrust.
So why is there this gap between the Falcon 9's performance and the Dragon's?