The Acetone is not actually "dissolving" the Styrofoam. Here is what is
happing. When styrofoam is placed in acetone, the long polymer strands
are dissolved, releasing the trapped air and causing the structure to
disintegrate. However, the insoluble crosslinked parts of the polymer
keep the polystyrene from dissolving entirely, in much the same way as
buoys on nets keep the entire net from sinking to the bottom of a lake.
Once the acetone evaporates, you are left with a hard crosslinked
polystyrene residue.
