The volume is 2 times greater when you double one of its dimensions.
The volume is 8 times greater when you double all three dimensions.
The math behind this is that doubling only 1 dimension means that you are multiplying by 2 only 1 time.
When you double all three dimensions, you are actually multiplying by 2 three different times (2 x 2 x 2) which would make it 8 times greater.
