tpad/mainframe.
Bit of confusion I think:
Baswell is correct regarding volume, at higher temperatures the volume of fuel is less.
What I meant: when hot, the weight of the same volume of fuel is less. But at cruise, the fuel cools down again, so the volume shrinks. And you can put the extra tonnes you put in the tail back into the mains because there is room again? (not to mention fuel burnt making space, of course)
That why you heavy drivers put fuel in by the tonne instead of litres.
Also: if the fuel didn't significantly cool, e.g. you go power line inspecting in a chopper on a 40 degree day, you actually have less endurance in the tanks then you would if you did the same job on a 10 degree day? Just because the fuel expands, doesn't magically give it more energy...