It can carry more cargo
The reason it didn't was because it needed extra fuel to carry out the docking tests:
The ATV is more than 32 feet (10 meters) long and almost 15 feet (4,5 meters) in diameter. It has a dry weight of about 23,000 pounds (10,5 ton). It docks automatically with the station, though station crew members can take charge of the process if difficulties arise.
It can carry more than 16,800 pounds (7,7 ton) of cargo. It can take to the station as much as 12,000 pounds (5,5 ton) of dry cargo, almost 1,850 pounds (830 kilo) of water, as much as 220 pounds (100 kilo) of gases, and up to 1,890 pounds (850 kilo) of propellant for the station.