Ok I just researched the difference.
The a330-300 and 200 are exactly the same size as the a340-300 and 200. However the a340 has a higher max takeoff weight, is faster, and more efficient. The a340 is actually loads more efficient than the a330. So there's yer answer.

@jamesPLANESii I it can hold more people but pretty much they're the same

It's not actually. @ThePlaneBuilder1775

@jamesPLANESii well the a340 is larger but they're pretty much the same.@ForeverPie

yep. @jamesPLANESii

@ForeverPie So you can't really mock them from looking exactly the same because they're supposed to be exactly the same.

@jamesPLANESii exactly

The airbus A330 and A340 are exactly the same planes, just with different engine orientations. And the A340 is more efficient.