Each Smooth-On foam has a number associated with it that represents the free-rise density or cubic foot "yield-per-pound" of the foam. Larger numbered foams have a higher density, whereas lower numbered foams have higher expansion rates.
For example, if you poured Foam-iT! 5 liquid into a cube measuring one cubic foot and the foam expanded to 12"x12"x12", it would weigh 5 lbs. If you pour Foam-iT! 10 liquid into the same cube, it would fill the same space and again expand to 12"x12"x12" (1 cubic foot). However, the Foam-iT! 10 cube would weigh 10 lbs. due to a higher density.