Usable fuel for the different versions of the AIRBUS widebodies : A330-200 : 139090 liters (36743 USG)
A330-300 : 97885 liters (28859 USG)
A340 : 139605 liters (36883 USG) and even a bit more (141500 liters) for more recent models.
The fuel burn rate of the 330 is a bit less than that of the 340, by about 500 Kg/hour average.
Now the 330, in a standard interior, has a dry operating weight around 125 tons, and the highest max take off weight I'm aware of is 230 tons, so it's not even able to take full fuel (110 tons) when empty. Add pax and cargo and soon this aircraft is restricted to around 80 tons of fuel, still enough to fly around 4500 nautical miles. The 340 on the other hand has a max take off weight up to 271 tons, so it can take almost full fuel and full payload at the same time, and be a really long range aircraft for missions above 6000 miles.
Of course that's probably the only reason to buy a 340, because on shorter stretches it is definitely more expensive to operate because of maintenace costs. Twice more engines, generators, oil pumps, fuel pumps, bleed valves......twice the maintenance cost for these parts.