I am a long-time Genii subscriber, and just came across this web site:
He sell coins which have been "shrunk" (radially compressed) using a standard industrial technique called magneforming. He can shrink quarters to 70% of their diameter, and can custom-shrink a large variety of coins to a desired percentage.
Seems like this could be useful for a very visual shrinking coins effect. Scroll to the bottom of the page to see limitations on coin types/compositions.