We will have the following:
When the rectangular solid is normal, the measurement can be set as side "a", side "b", and side "c" [With these 3 measurements we can construct a rectangular solid]. So, the volume would be given by:
Now, if we were to double each dimension [ie side "2a", side "2b" & side "2c"] we would have that the volume would be given by:
So, the volume would have increased by a factor of 8, while the sides increased by a factor of 2.