Here's a foolproof and very safe method for shrinking Styrofoam objects (no toxic fumes!). Unfortunately, it is a little expensive.
Cups are now shrunk to size of a thimble
By the way, what's happening scientifically is that the increased water pressure at depth in the ocean compresses the Styrofoam, and forces out the air. The deeper you go, the higher the pressure.
Science is so much fun!