I agree.... shrunken quarter. Neat to find it in circulation! http://www.popsci.com/diy/article/2003-10/making-small-change-smaller
That's quite interesting, I had never read of such. To the OP and anyone else, think of it this way: If the quarter die struck a dime planchet, the complete design would not be depicted.
By that "technical" definition, new jeans don't "shrink" when you wash them, either. You're welcome to refer to them as "thickened" or "enprolated", but I think you'll have a hard time getting it to catch on...