Dissolving polystyrene in acetone (or any other organic solvent) essentially removes all the air form the polymer, and you are left with the styrene monomers. If you can evaporate ALL of the acetone, you will have a product that is pretty firm but not very heavy. These terms are relative, so it depends on how you define firm, and heavy. Once it is dissolved, you have a white, gooey, material which you can shape into a ball, or a brick, etc. At this point, it is still pliable and soft. As the acetone evaporates, and it may take a long time for it to evaporate from the inside of whatever structure you made, it will be firm, and no longer pliable.