Why less than half a ton of supplies?
It’s possible they are starting small and will be increasing the payload weight on each successive flight. The CRS-2 launch will using Dragon’s trunk.
“CRS-2 will debut the use of the Dragon’s trunk – used for unpressurized/external cargo, that will utilize Dextre for removal operations (see image left) - with payloads already allocated for a ride uphill in the trunk (L2 Link).”
http://www.nasaspaceflight.com/2012/09/spacex-october-7-dragons-opening-crs-mission-iss/